Intro

The Sound

A good front panel layout allows a simple crossover.
Basic simulation came very close to this, only L2041 and R1011 (as always) had to be adjusted by ear.
L2021 is a high-ohmic coil, DCR = 1.2 ohm. C2021 is made from 3 x 22 uF in parallel. The C-coil for bass (L311) allows extremely low DCR.
R2031 and R1011 are the resistors for tweaking midrange and tweeter level. I had a hard time deciding between 1R8 and 2R2 for R2031 but ended up with 1R8.
For R1011 3R3 was soon in place. 2R2 renders a rather flat response but makes a too aggressive treble level. 2R7 if you like lots of detail. 3R3 if you want a balanced presentation of basic notes and overtones.

If you want to make a bass reflex cabinet, go for 40 litre and a port tuning of 35 Hz.
Add minimum 2 braces to the cabinet. Not further details from here but keep frnt panel design.
Port = 68 Ø x 120 mm length. And it can be front or rear.

SketchUp file available here.

Go to cabinet construction page

Measurements may give us an idea of tonal balance of a system, i.e. too much or too little energy in certain areas. 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 us 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 anything 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 way 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.

Left: SPL @ 1 meter, 2.8 volts, merged with bass nearfield response at 250 Hz. System sensitivity = 88 dB/2.8V/1 meter.
Right: System impedance. Minimum = 3 ohms at 150 Hz. For the newbee this means a 4 ohms speaker and your solid state amp will have no trouble driving them. For valve lovers, you need minimum 50 watts push-pull, preferably 80-100 watt, to fully exploit this speaker.
Low-wattage SET amps are not recommended.

CROSSOVER LAYOUT
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Since introduction, Alumen-Z and MKT Z-caps have found their way into the crossover.

Place the bass crossover on the rear panel behind the bass driver.

Right: Stuff the mid cabinet vent with acoustilux as seen on photo to make an aperiodic tuning.
Cut a piece 135 x 150 mm of 30 mm acoustilux, roll it and stuff the vent. (135 mm = length of vent).
I had some grey acoustilux as seen on photo. It is not grey felt.