I can’t imagine a situation where low xmax or weak motor are positives. Raw driver LF response doesn’t strike me as meaningful in this century either. Maybe when frequency shaping tools required an MSEE to implement rather than a program and a finger to run on the trackpad it was important.
Basically IMO you always want the least passband resonances, longest throw (assuming inductance is kept low and linear), and strongest motor you can get. System design will get you the rest of the way.
Do you have experience building/architecting open baffle designs?
I ask as I am going to try building some and want to know if you're on point here or not.
Many, many systems (and I know DIY is a real mixed bag of opinions and results)that are purported to be nice use the "Goldwood, Eminence" style 15-18 woofers with lower xmax and higher(.5-.8) or even quite high(1.0-1.5) Q. Some of course use a different approach such as Linkwitz using 10" low Q(.30), Higher Xmax woofers in the LX521, or the GR Research high Xmax options.
The Goldwood and GRS drivers are very affordable and lets face it 3 or 4mm of Xmax on a 15 or 18 is significant air movement, let alone considering using multiples.
(Even if I just consider 2mm the practical limit for these drivers due to potential distortions when driven "hard".)
SB Acoustics came out recently(1yr ago) with a couple of higher Q, High Xmax beefy units for Open Baffle design. They look quite nice.
Open baffle loudspeakers are extremely popular for good reason. They can be fitted with large low frequency drivers and they sound "real," in part because open baffles play with room reflections and sound good even off-axis. A big concern though, is control over the bass response, where the...
audioxpress.com
The main problem I currently see with high xmax woofers is cost as high xmax requires big motors/magnets and typically ends up with low sensitivity so you need several fairly expensive drivers. (Though the low xmax, GRS 15's I picked up are lowish sensitivity as well for an 8ohm, 15" at 87db, but I will get a high q spl bump )
So the advantage of "weak motors" is cost savings when SOTA is not the game.
With the "Goldwood" approach you still need several drivers due to excursion limits but the drivers are inexpensive and one can buy 4huge 15-18" woofers for the price of one high xmax 10 or 12" unit. (except now the SB Acoustics new line is fairly affordable though still pricier)
Very high QTS can help compensate for the acoustic roll off with it's bump and therefore requires less power from the amps and so the xmax advantage of High Xmax/Lower QTS fades as it depends on much more power to accommodate the acoustic fall-off and more EQ boosting. So all the trade offs play out though I have no idea yet what is best myself. I am under no illusions that the GRS drivers are a pound for pound substitute for better units, they are just darn affordable.
I just ordered 4 GRS 15" woofers to play with in basic H frames and they were $125 for all 4 after discounts. That is crazy cheap. (-Unless they sound terrible and then it is just crazy. )If I like them and need more bass, 4 more is $250 total for 8, 15" woofers. The woofer Linkwitz is using in the LX521 cost that per driver (actually $330 or $1320 for 4,10"ers. 8 would be a huge layout of $2640 barring a discount).
Anyway since I am obviously (at least to me) doing this for fun and not SOTA then the $$$ layout is easy to accept for now until I get into Dipole or not. And who doesn't want 8, 15" drivers exposed in their living room??
Amp power is cheap if you are cool with the Monoprice 605030 for testing like I am. That said, the GRS based system won't need much power to play at a pretty reasonable SPL. I should be able to drive all 4 (or 8 if I get crazy) woofers easily with that $100 amp and have room for plenty of EQ/DSP adjustment.
Some other system with robust low qts drivers could require multiple times the power.