DTQWT, 10-inch or 12-inch?
Copyright 2012 Troels Gravesen 

I've had this question a number of times I'll try to put together the things to consider before making your final choice. Below I've copied in the basic principles of DTQWT from the DTQWT mkII file and here's a link to the page on Amps for DTQWT. The TQWT and DTQWT were made to suit low-wattage valve amps to fit my preference for these amplifiers. It doesn't mean they can't be driven from almost any amp but we have to know what we're doing.

First of all: The 12" was made to play deeper, not louder. I cannot stress this enough because we inevitably regard a bigger speakers as one that should be able to play louder. This is not the case for the 12" version. The 10" version features a 40 Hz horn (check impedance curve) where the 12" version with its more than 3 meter long horn takes us down to 30 Hz. The 8008 front driver carries the full signal and is part of the speaker's bass performance. Thus, the 8008 front driver does not have a high-pass filter, which has indisputable advantages but also limits power handling. The OBL-11 speaker basically having the same drivers and crossover can play louder but requires expensive coupling capacitors for the middriver to perform the best. As always: Any speaker is a compromise between a set of design parametres.
I've seen a 8008 cone ripped apart from using 400 watt solid state amp - or lending the system to childrens' disco party. Speakers having 95 dB sensitivity should simply not be fed 400 solid state watts. The voice coil was fully intact but the cone was just simply ripped apart. The other reason for this was that the speaker was supposed to energise more than 100 square metres room - or close to 250 cubic metres. This is equivalent to a smaller cinema space - and this is a completely different story. Before building speakers we need to analyse what kind of sound level we're aiming at, what is the room size - and volume, and what kind of amplifier are we going to use. Any burned speaker is a misused speaker.

I've more than once made guests pretty uncomfortable by playing the first movement of Mahler's 5th at loud level, so don't think the DTQWT can't play loud. It can (in my 25 sqm living room), but we need to analyse our room and sound level requirements before deciding. And this is where I cannot help because I can't know how loud you play. 

The basic sound of the 10- and 12-inch version is the same due to the front drivers being the same for the 12" version and so is the crossover. The wider front panel of the 12" version may add a little to the low-end weight of the 8008 driver, but not much. Not all music contains much information below 40 Hz, but the deeper frequences we can reproduce the better we get information of the room in which the recording was made. It adds to the overall perceived soundstage.
As discussed in detail here, the price for high efficiency plus low-end extension is size. Deep bass and high efficiency means huge cabinets
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In principle the 12" version should display reduced distortion due to the larger size of the bass drivers and reduced cone motion and so it may be, but this is not the reason why you should choose the 12" version over the 10" version. The bass drivers of the 10" version moves very little even at seriously loud levels and distortion is not an issue. And in principle we should be able to produce the same low-end extension from the 10" version by simply extending the 10" cabinets to 1.5 meter height. I never tried this but my guess is that it will not sound exactly the same as the 12" version anyway. Large drivers have their way of loading a room that smaller speakers can't.


Left: 12" Eminence Deltalite 2512. Right: 10" Deltalite 2510 drivers.


Basic principles of DTQWT:

DTQWT is in principle a 2-way with integrated subwoofers, thus no high-pass filter on the main front driver. This has the indisputable advantage of not needing any series capacitors, which would be excessively expensive if of good quality and sonically inferior to no caps, no matter how good the caps are. 
Loading a horn helps keeping front driver membrane in place due to high acoustic impedance compared to a vented enclosure or open baffle and this is further assisted by the mode of action of all drivers.
DTQWT features an 8" front driver and two 10" rear mounted drivers, all connected with positive polarity. They are placed opposite to one another and the rear drivers are fed through a huge coil, thus only adding weight to the lower octaves. All drivers load a center horn and you may ask whether the large bass drivers won't have an impact on the front driver from sharing the same center horn and the answer is yes - and this is intentional. Air goes where pressure is low, thus when the rear bass drivers move inwards, they will push air into the large center horn and by doing so they will suck air from the front horn and pull in the membrane of the front driver. Obviously the front driver will do the same thing for the bass drivers. 
By fine-tuning expansion of front, center and rear horns while measuring impedance, a balance can be found allowing the best possible power handling of the main front driver. The subjective experience from this is a cleaner midrange, distortion has been lowered (see measurements below) at high SPLs and yes - we have more bass. In fact a lot more bass compared to TQWT. The typical quality of the bass from a horn is quite different from conventional high mass/low efficiency bass drivers in small vented enclosures. The DTQWT delivers deep, dynamic and dry bass without the typical vented enclosure overhang.
In principle we can load the horn with any 10" bass drivers regardless of Fs, but there are limits. I found drivers with Fs ranging from 30-60 Hz suitable and without any noticeable difference unless the driver at the same time has high Fs and extremely low Qt, e.g. Faital 10PR300, which on paper looked suitable but in reality didn't perform well. Actually Fs was closer to 65 Hz and Qt = 26. The Eminence Deltalite 2510 II proved just ideal with Fs = 49 Hz and Qt = 0.39 (own data).
The length of the horn determines the lowest frequences you can expect from these kind of systems. This is not a function of the driver's Fs as many may think. Mind you: You can have a 5" high-Fs driver doing 30 Hz; you only have to built a humongous exponential horn to make it happen.
DTQWT is designed to do 40 Hz and from the impedance graphs below you can see this is the tuning frequency of the horn making two impedance peaks like a vented system.

Despite the description above, many people have trouble understanding why we don't need bass drivers with huge Xmax and low Fs. Please compare the combined membrane area to an e.g. 10" bass driver in a vented cabinet with the DTQWT. Here we have some 2 x 330 cm^2 from the bass drivers and 220 cm^2 from the front driver, in total 880 cm^2. This is 2.6 times the membrane area of a single 10" driver. If a single 10" bass-reflex loaded driver has to move +/- 5 mm, we only need 5/2.6= +/- 1.9 mm.
Next, all drivers in the DTQWT loads a horn, an acoustic transformer. Wikipedia: A horn loudspeaker is a complete loudspeaker or loudspeaker element which uses a horn to increase the overall efficiency of the driving element, typically a diaphragm driven by an electromagnet. The horn itself is a passive component and does not amplify the sound from the driving element as such, but rather improves the coupling efficiency between the speaker driver and the air. The horn can be thought of as an "acoustic transformer" that provides impedance matching between the relatively dense diaphragm material and the air of low density. The result is greater acoustic output from a given driver.
The DTQWT horn works quite different compared to a bass reflex enclosure and we do not need excessive panel thickness and I wish you could all experience how the e.g. side panels feels when I play bass even at thunderous levels. You won't feel much.