Well, it had to be done. Mating a large bass driver with a large midrange dome. Nothing new under the sun, but very little used. I guess Acoustic Research was the first to do it with their legendary AR3a speaker featuring a 12" bass driver handing over to a 1½" mid-dome from around 575 Hz according to specs. I never heard the AR3a, but a friend of mine still have his AR11s with pretty much the same drivers - and it does well - after I re-foamed his bass drivers.
Best known today for this layout of drivers is ATC with their well regarded range of studio/home speakers featuring the magnificent 3" double spider mid-dome. British VOLT produce a couple of (single spider) mid-domes, one 2" and one 3". Most recent addition to the limited range of mid-domes is the SBAcoustics MD60N-6, which I tested in my SBA-7MD construction - and I very much liked what I heard. I just learned Yamaha launched a new speaker with a 3-1/4" mid-dome, so maybe we'll see more mid-domes in the future - and I certainly wouldn't mind.
Last year ScanSpeak released their D8404/552000 mid-driver, a one-of-a-kind mid-dome with elliptical voice coil of 84/68 mm - and double spider. Never seen before and partly due to corona I spent almost two years with the prototypes before final release at the Munich HighEnd show. And suffice to say, I had some good times with the "D84", which resulted in the Ellipticor-A50-mkII.
Now, in mating a bass driver to the D84, we should most likely not look for some heavy subwoofer type, rather a driver with a more light-weight cone with the ability of playing some midrange, possibly all of the midrange, although we're not going to take advantage of this.
I have two 83 liter test cabinets and here the merged response (200 Hz) of the 38WE driver:

This 15" driver works really well up to around 1200 Hz. More than enough for our purpose with an intended point of crossover around 375-400 Hz. Based on preliminary tests, we can achieve a system sensitivity around 92-93 dB. On an infinite baffle we may hit some 95 dB, but this is of no use in real-world cabinets - even with some room-gain. Room gain is the necessary evil and as a designer I can only guess where the speakers will be placed. The Ellipticor-84 is intended - like all my speakers - to be somewhat out on the floor having modest room-gain, maybe 1-2 dB.
At 400 Hz we have a wavelength of some 85 cm, so phase issues (time-alignment) is of minor importance between bass and mid-dome and the D24 and D84 has very much the same acoustic depth, so we can use a flat baffle like the A50.

The two mid-domes look like this and we have a suitable range to below 300 Hz and up to around 5000 Hz, some 4 octave. Nice!

Below you can see my workshop mockup and I really wasn't prepared for my 32 wpc tube amp to deliver the dynamics I heard. It can play insanely loud from only 32 watts and I guess the high impedance in the bass partly counts for this. The crossover is different from my A50s - but not much, just found an easier way to roll-off the D84 towards the tweeter. Doing the basic measurements and crossover simulation took some time, but here the D84/D24 compo driven from crossover:

I looks smooth - and it sounds smooth. Well, really not the place here for showing measurements, but these initial readings are crucial for getting ideas of what we can achieve from a selection of drivers. As always, measurements and simulations tell us what may be possible, but only practical experiments tell us what has any future.

Above a recent selection of albums, which were used during testing - together with more well known albums. Insert is the Dream Box by Pat Metheny as I recently was fortunate to attend his Side Eye tour here in town. What a concert, more than 2½ hours with new music and old themes pupping up in new dress. When I walked home from the concert, I felt privileged to have been able to follow PM since around 1980, where a friend pointed to an album: Bright Size Life. It changed my musical path forever.

Funny thing: Recently I had a mail suggesting that when I write very little on sound on an actual speaker, the writer would interpret this as having reservations on the sound. He also noted that on one occasion I drank an expresso and on another occasion whisky - and took that as a sign of preference. I can ensure you that there is no correlation between my writing - or drinking - and how much I enjoy an actual speaker. None whatsoever! I always drink expresso in the morning and maybe whisky late night.

But there shall be no doubt that I like this speaker very, very - very much! And I assume you have already read this page carefully: http://www.troelsgravesen.dk/choices.htm
And if you really wanna know: Extremely dynamic bass, smooth and transparent midrange and treble. You can put any state-of-the-art equipment in front of these speakers, and they will deliver the goodies - assuming good recordings.
So, there you have it!

Stop press: Some 8-10 years ago I used the Jazz at the Pawnshop extensively for testing my final speakers/crossovers and I really hadn't use it much since then, maybe even several years since I heard it last. I'm Confessing was one of my favourite tracks and when I came to the clarinet solo I had to stop, make me an expresso, and go back again. I really never heard the solo this way, free of some edginess that had always been a signature of this particular instrument. Smooth on the ear as never before. Well, my gear is better than it was 10 years ago, so that may be count for some, but I guess the wide coverage of the D84 is the main responsible. I found my other copies of the Pawnshop, I have an original pressing back from the 80'ies - same thing. Smooth as never before. Which, by the way, tells us that auditory memory may leave a lot to be desired, but some apparently stays put and will never be forgotten like some tastes and flavours.
I went to hear the same track on my A50 speakers - same thing. To some extent they share the same crossover topology between D84 and D24, so not a big surprise. This D84 does its thing when treated properly from your front gear.

If you want to have an attenuator on the D84 dome, add the Fostex R80B. Using the attenuator, we change R6 to a lower value and attenuator must be at -1 dB to make the same level.

Cabinet with tilt of bass baffle. Obviously you don't need two ports, but you can place it to the rear - or bottom. Wherever I place the port, there will always be a guy who wants it different!
Now, you can make the cabinets classic with a vertical front panel to easy construction, but having the 38WE working up into the lower midrange the bass baffle tilt does good for the overall projection of sound.
I used 25 mm HDF for the front panels.
Cut the supplied 100 mm tube to two pcs of 90 mm for the port.

Everything starts with a mockup!
I have some 83 litre cabs for testing 15" drivers. That doesn't make the bass go as deep as can be, but they are more than suitable for a start to hear if this thing has any future at all.
The preliminary crossover was rather complicated - as often is in the beginning. Then the process of simplification starts. What can be made smarter, what can be left out and still deliver. Many times you go to bed thinking this is it, but after a good night's sleep you get new ideas to how things can be done. You hear more records and discover things that might sound better and you do modifications and adjust mid-dome and tweeter levels.
What struck me first was how deep, potent and dynamic bass this construction delivers driven from "only" 32 watts from my EAR-861. No bi-amping, just 32 tube watts. My goodness this amp can rock these light-weight 15" bass drivers. The 91-92 dB system sensitivity does its thing. The Kuzma, MSL cartridge, the EAR-868, the EAR-861 - and then this Ellipticor-84... This, for sure, would satisfy most listeners, I'm sure. OK, not a cheap system, I admit. EAR prices have skyrocketed over the last couple of years. I was lucky a few years back to purchase the 2nd hand -861 for less than 3k€.
Next thing that surprises - again - is the mid-dome's ability to play loud without distortion. Saxophone, violins, electric guitars, etc. Everything clean and loud till your ears bleed.

Test crossover.

To my wife's horror, testing my mockups in our living room!
No, she didn't freak out! Bless her tolerant soul.
The speakers certainly didn't do less here, driven from my bridged EAR-861s. Wauw!
After that no doubt in my mind - go to the workshop to cut panels.

Making 125+ liter cabinets is a bit more complicated than a 20 liter standmount!

All braces done and have started gluing.
Before gluing the cabinet sides and rear I first glued panel E and B. That done I glued panel A to E+B making a structure that can slide in and out if the cabinet.
Next I glued panels C and D, also able to slide in and out of cabinet.
These two structures make a good scaffold for assembly of the entire cabinet.

Above the bitumen pads in place. Midrange cabinet and upper braces can still slide into the cabinet.
Bottom braces must be added during final cabinet assembly.
Remember bitumen pads do not have to cover the entire surface of panels. 5-10 mm to all nearby structures is fine. 2 sqm provided with kit if you want to use this.
http://www.troelsgravesen.dk/tips.htm#Glue_for_Bitumen_Pads

Top structure added bitumen pads on rear side of midrange back panel. Right: With felt.
No bitumen pads inside midrange cabinet.

Cut all felt sheets and mark them carefully for placement.
And felt cuts do not have to be in single pieces. To utilise the felt properly a patchwork is fine.

All felt sheets applied.
No felt on rear panel above port hole, where the mid crossover will be.
At the bottom next to the port hole, leave some panel, like 6 cm, blank to give room for the bass crossover. There is just about space for the crossover. See at bottom of page.

1-2: Cut 8 pcs 35 x 50 cm and fold two like this:

- and place in compartment 1 and 2.
Don't fasten, just let the "pillows" rest on braces.

3: Cut 2 pcs 18 x 112 cm and add to sides and bottom of compartment 3. See images above.

5: Cut 2 pcs of 19 x 83 cm and place on sides and rear panel of compartment 5. This covers the crossovers for mid and tweeter.
All in all around 550 cm x 50 cm = 2.75 sqm. 3.0 sqm supplied in kit.

Using countersunk screws will eventually skew the drivers a tiny bit and if you have to press the decor ring into place - you are never going to get it up again!
So, add some foil around the driver when fastening the screws. This way you will leave some 0.1-0.2 mm space between decor ring and cabinet. Removing the decor ring by rotating until it slips the magnets and lift with another small neo magnet. The tweeter decor ring pops up by itself. The D84 doesn't.
When routing for the D24 and D84, make the rebate diameter +0.5 mm. See Ellipticor-A50-mkII for more pics.

The screws for the tweeter need to be well countersunk. I found some good torx 20, 4 x 20 mm screws at the local Bauhaus and for the tweeter I did some grinding to reduce the head size and to make sure the decor ring doesn't ride the edge of any screws and cause buzzing. This is important.

So, cabinets waiting for the crossover parts from Jantzen Audio.
In the meantime I'll start making the stands. See below ->

A few comments on MEASUREMENTS before you start interpreting the readings below.
First of all, if we think measurements will tell us how a speaker sounds, we're wrong. The perception of sound is way too subjective to be reflected in any measurements we can perform. A loudspeaker system is meant to give us a satisfying idea of an acoustic event and for some people a pair of 5 USD ear-plugs are enough, others spend 200 kUSD on a truly full-range pair of speakers - and the latter may not be happier than the former.
Measurements may give us an idea of tonal balance of a system, i.e. too much or too little energy in certain areas, although dispersion characteristics play a vital role here. A two-way 7+1 and a three-way 7+4+1 may display similar horizontal dispersion, yet sound very different. Measurements may tell us about bass extension if far-field measurements are merged with near-field measurements. In addition to this, ports may contribute to bass extension. Most of we diy'ers do not have access to an anechoic room for full-range measurements from 20-20000 Hz.
What cannot be seen is what kind of bass performance we get in a given room. Bass performance is highly dependent on in-room placement of your speaker and the same speaker can be boomy in one place and lean in another. Actual SPL level at 1 meter distance and 2.8V input is useful for en estimate of system sensitivity and combined with the impedance profile may give an idea of how powerful an amplifier is needed to drive the speaker to adequate levels.
What measurements do not tell is the very sound of the speaker unless displaying serious linear distortion. The level of transparency, the ability to resolve micro-details, the "speed" of the bass, etc., cannot be derived from these data. Distortion measurements rarely tell much unless seriously bad, and most modern drivers display low distortion within their specified operating range.
Many people put way too much into these graphs and my comments here are only meant as warning against over-interpretation. There are more to good sound than what can be extracted from a few graphs. Every graph needs interpretation in terms of what it means sonically and how it impacts our choice of mating drivers, cabinet and crossover design.
What measurements certainly do not tell is the sonic signature of the speaker, because speaker cones made from polypropylene, aluminum, Kevlar, paper, glass fiber, carbon fiber, magnesium, ceramics or even diamonds all have their way of adding spices to the stew. Nor do measurements tell what impact the quality of the crossover components add to the sound, from state of the art components to the cheapest of coils and caps, they all measure the same if values are correct, yet sound very different.

SPL of bass driver without the crossover. Port contribution not included.
These 38WEs go deep!

Final system impedance of both speakers - if you can distinguish the left and right speaker!
That is an easy load - no wonder my only 32 wpc EAR-861 loves these speakers.

All kit and component prices may be subject to change and are always to be confirmed by Jantzen Audio Denmark.

Placement of crossovers in cabinet on rear panel. The reason for not placing the mid-tweeter sections in the mid-tweeter compartment is to get the crossovers as far away from the D84 magnets as possible. Believe me, the magnet structure of the D84 is extremely strong.
The critical capacitors are all in alu tubes, thus protected from vibration.

Place bass crossover next to the port and mid+tweeter crossovers in middle section as seen on photo. This to keep coils as far apart as possible.
Fasten boards with screws.

Cover bottom panel with acoustilux and add some leftovers next to the port.
Cover mid-tweeter section as seen on photo.

STANDS

The stands can be made in many ways as long as we reach a hight of around 23-25 cm. I made my stands from 20 mm BB - and as I had 2 sets of Soundcare spikes on the shelf and decided to finally use them.
1 meter M8 threaded rod was cut into 16 pcs 45 mm bits and I glued them with epoxy as seen on the drawing. I wanted them not to be visible from the outside.

For the Soundcare spikes I bought some M8 x 30 mm threaded inserts. Threaded hex nut, I think it's called (bought at Bauhaus). These were 14.5 mm on their widest spot and I trimmed them a little on my bench grinder and drilled a 14 mm holes and gently hammered them in. No epoxy glue used here.