Since you paid a fortune for the Bliesma tweeter to enable a lower crossover frequency on the tweeter, I suggest increasing your midwoofer size and quality to get better low mid dynamics and lower distortion.
From SB Acoustics one of these MW19P-8 drivers looks ideal for a two way.
http://www.sbacoustics.com/index.php/products/midwoofers/satori/7-satori-mw19p-8/
Too bad they don't have a midrange version like they do for the 6" midwoofer.
For a three way mid, I suggest looking at the Acoustic Elegance TD8M:
http://aespeakers.com/shop/td/td8m/
If you want to be unreasonable about enclosure quality, I suggest building your bass enclosure in a rigid B&W Matrix-like structure and damping your midrange enclosure with Hawaphon.
https://www.korff.ch/en/hawaphon-eng/
IF you want to be really autistic, I suggest lining your listening room walls with Hawaphon as well, especially if you live in the city.
Thanks for the suggestions.
But as far as I can see now the SB Acoustics SB17NAC35-4 (and SB17BNAC35-4) midwoofer seems to be of top quality in a 3-way?
I'm thinking I can take it to 120 to 150Hz LR4.
The reasons I chose it are because it has a very high cone breakup of around 10kHz well outside the most sensitive area of our ears so any residual distortion falling on it are not going to be audible, also the ringing isn't that bad. Compared to for instance a Seas W18EX001 which breaks up around 5kHz which is much more audible and closer to the crossover freq of ~1500Hz and it rings much more severe I think amplified HD will be more audible there.
As far as I can tell the good aluminium coned midwoofers are a fair bit tighter in time / CSD than the best paper cones who always have very minor breakup in their passband range visible in CSD's. Also the harmonic distortion of the SB17NAC35-4 is the best in the entire range of SB Acoustics. The Satori range does not give better performance there. There's no getting lower distortion anywhere for 200Hz up, I looked very carefully at this also at much more expensive 8" drivers (and even some 10" drivers).
As for dynamics in the low mid range. Yes it can be a bit better in the 120-200Hz range with some 8" or 10" drivers but I'm actually not going to use them very loud as I'll be using them nearfield. Also if I were to do a 150Hz crossover it's down 6dB at 150Hz and almost 3dB I think at 200Hz further helping. In any case for my use the distortion is still very low in this area and I expect no compression at my volume needs, and if need be when with a 150Hz crossover I can play the mid driver over the entire range I use it at almost 120dB SPL volume at 1m (both speakers at once and in linear coil travel of the drivers, even higher SPL for maximum coil travel) that's louder than I need but it indicates the reserve I have even in the lower mids. So all in all it seems like the best driver for my project despite its low cost.
Here some measurements of it and the related (but slightly less well measuring) 8-ohm version:
http://www.audioexcite.com/?page_id=5833
http://www.troelsgravesen.dk/SBAcoustics-61-NAC.htm
https://hificompass.com/en/speakers/measurements/sbacoustics/sb-acoustics-sb17nbac35-8
I also have 2 measurement sets of it in hobby hifi magazines which I can't show here but are equally well.
A two way with MW19P-8 as done by hificompass
https://hificompass.com/en/projects/2-way-systems/pharaoh measures worse, my speakers should be better in many ways also in the mids. (also due to better baffle, better DSP crossover, better dead cabinet and due to it being closed 3-way also better in the bass). (oh and hificompass even reported problems themselves in audibility of mid-tweeter differences and trying different crossovers to lessen this)
edit: oh yes and one more reason to use a 6" driver is that the mid and tweeter can be closer physically giving a near coaxial/point source radiation / no lobing with the low crossover freq. This is less so with an 8" driver. Also the SB17NAC35-4 has almost no freq drop around 1500Hz off axis while many other 6" drivers and all 8" and larger drivers have significant off axis dropoff at that freq further degrading off-axis performance. I think overall the SB17NAC35-4 is really the perfect match for the T34B-4 with 1500Hz LR4 DSP crossover. There shouldn't really be an audible error in any area I can see with my knowledge so far.
The Hawaphon hmm I'm not that impressed actually.. I believe 5mm leadbitumen or a mass plate gives about the same result? I'll be using 19mm MDF + 19mm birch plywood + bracing and then 5mm leadbitumen on both the inside and outside of the speakers (won't be the prettiest but I don't care at all about that
and decided to do the inside stuffing with fibreglass (cheaper than wool with at least equal results but for the fibreglass I know the exact gass flow resistance value and don't know it for wool)
And about my room not too worried about the neighbours. Only have one neighbour who I could possibly bother but it's hard to do so, live in an old city building with massive walls and the neighbour is gone during the day. Also my walls are isolated and I'll be adding 27cm to 45cm thick Knauf Acoustifit to the walls for creating my near anechoic space
(only bass won't be anechoic but where not absorbed it can either escape into a larger space or when it is reflected very close to the speakers it will be in phase)
But thanks for the suggestions again though! This was about the last moment for changing my mind on things, starting building the studio next week and the speakers right after