Very interesting discussion.
Seems the MM111540s have an XMax of 11mm, not 25. Xmech (Xmech x 2?) is probably 25mm.
"Called Polk Audio and just confirmed through their engineering department that the FULL THROW PEAK is 22mm's. It's 11mm from center forward and obviously 11mm's from center backward."
The other thing I don't like about the MM1540s is that the speaker face "cone" is nearly flat and it doesn't appear to be reinforced with much/anything. I think that invites distortion of the speaker face "cone" surface at high acceleration/air loads, which will distort the sound wave.
I bought an Infinity 120.9 subwoofer to play with. Its got a reinforced cone that is really a cone, an efficiency of 91dB and an honest Xmax of 17mm. It should move as much air as a 15 that has a smaller Xmax. It was $130.
The 120.9s don't have the power handling you guys are talking about, but I'm not aiming for the SPL you guys are shooting for either. I've used Infinity components in car stereos before and like their approach and performance, particularly the SQ. One of the only car audio companies to provide TS parameters for all their products.
If I need more power/ SPL, I'll use multiple drivers. If I need a lower Fs, I'll add a passive driver. To start with, I am going to mount a single 120.9 in a 1 ft^3 box and see how I like it before I see how it models on paper. It will be much better than what I am listening to right now.
I've done a bit of sound engineering for bands. When it comes to woofers, I'm all about Xmax and cone rigidity. All the other limitations can be overcome with multiple drivers, box size, tuning, etc. Of course those things costs more.
I'm on a budget too (time more so than money), so I'm going to start with something simple and experiment. $130 for the speaker, $20 for the box, I'll throw a table cloth over it to cover it up, use the sub out on my amp and have a starting point for comparison. 100 watts is 20 dB, 91+20 = 101 dB (@ 1 meter, no room effect), lets see what that does for my bass thirst. I'll go forward from there.
FWIW, Behringer has some crazy good deals on subwoofers, powered or unpowered. I haven't played with them yet, but I like their powered subs in that they match the amp to the speaker and do a bunch of safeguarding so that the sub is protected.
This is an interesting thread.
I love his box design. I'll probably end up with something similar using a couple 12s.
Caveat. I have no affiliations with any of these companies, nor do I really know what I am doing.