Some more updates, starting with bass:
I measured my previous design to see what kind of group delay I was getting. The speaker is tuned to 47Hz, high passed at 40Hz, with ~12dB of bass boost that makes it perfectly flat to 45Hz. Unfortunately, the speaker can reach 2 - 2.5 cycles at and below tuning. This is clearly not good, at least by the textbook definition.
EDIT: One complication is that the SMSL amp I used to power the woofers seem to have a built in bass boost. The model shows I need up to 12dB of boost, but I only applied 9dB on the miniDSP to achieve it. This might have made the group delay a lot worse as the results are a lot worse than what WinISD predicted. One explanation could be that there were a total of 5 filters used (4 on the miniDSP, 1 in the amp) to shape the bass response. Would using 4 filters instead of 1 in the miniDSP worsen the group delay? I used 4 when I could have just used 2 (boost and high pass) because I was anal on getting the bass to be literally ruler flat. However, if more filters cause more delays, I will reduce the number of filters.
That said, my subjective opinion of the bass quality is excellent. Maybe it's because I might prefer a boomier sounding bass, because I don't like the sound of many well regarded SQ subwoofers from the likes of JL, Martin Logan or Paradigm. Because of this finding, I will continue my previous plan to boost the bass flat despite the group delay problems.
The second update is on the tweeter: Originally I switched out the Scanspeak tweeters because SME pointed out that their very low 85.9dB test bench sensitivity will hold back the performance. However, after some research, it turns out that it seems every tweeter that Medley's Musings test has 4-6dB lower sensitivity than the manufacturer's rating. Scanspeak is known to have accurate, if not conservative specs even with Klippel testing. Two other tests for two different tweeters in the same tweeter family showed that Scanspeak's sensitivity rating is actually 1dB conservative. Therefore I believe the D3004/602010 tweeter I previously used does have an honest 90dB sensitivity.
As for the Wavecor tweeter, I noticed the sensitivity rating is an average between 5-20KHz. However, the tweeter exhibits a rising top end, which means that the actual sensitivity is actually 1-2dB lower because of the lower 2-5KHz band. Therefore, the Wavecor tweeter is only at most 2dB more sensitive instead of 7dB that I previously thought. The Wavecor tweeter also has poorer off axis response partly due to the bigger Sd. The sound quality is also a gamble. The tweeter is also 10mm bigger, increasing CTC distance. Therefore, the Wavecor lost of its advantage in sensitivity and the disadvantages now overwhelm the advantages. The Scanspeak tweeter is back because it is smaller, better off axis dispersion, better sound quality, and having a protective grill.
Because the Scan is back in the game, now the question is, should I do a true Ricci style build and upgrade to the beryllium version of the tweeter? What do you guys think? The price is $120 vs $280 for each tweeter. However, I just read a measurement of the Be tweeter and the measurements and especially the distortion data is simply outstanding. It would have no problem handling the 1500Hz crossover. The tester also mentioned that besides having less sensitivity, it measures and sounds essentially the same as the $450-$520 normal sized Scanspeak beryllium tweeter as they share the same diaphragm, just a weaker motor on the small one.
The devil's advocate side of me is saying no, at ~$2500, this silly speaker is already stupid expensive and way more expensive than expected. I also want to hear the difference between the old and the new and how audible the difference between the two are when they share the same mid and tweeter, but this one will have an improved crossover, diffraction control, DSP, amp, midrange damping and construction.
Link to the Scanspeak small Be tweeter measurements
This is the distortion measured at 98dB at 1 meter, barely any distortion even at 1000Hz.