Ricci, I took my MM1540 DVC out and measured xmech - it's a bit over 50mm p2p. The way the driver is designed, the voice coil will never hit the back plate. I don't know what the real-life xmax is - I do know that deciphering the proper specs for that driver based on Polk literature was nearly impossible and it would be great to have proper, empirical tests done. The shallow mount and light weight really do offer some intriguing installation options - for example I found the MoMo performed quite well in a dipole sub configuration.
I don't feel the discussion of the MoMo 15" on that thread you linked to was an accurate reflection of what to expect from that driver. There was a whole lot of speculation from folks who've never seen or heard one. I'm as guilty as anyone on that thread of pushing inaccurate specs for that driver. Shame on Polk, I say. Even the power handling is listed differently on their web site vs. their manual for the DVC version - Polk's website claims 425 watts RMS vs. 360 listed in the page from the manual posted in that thread - Measurements aside, I find it to be a very good sounding driver as long as it's not pushed to extremes (50mm p2p excursion). Definitely capable of stirring up the air at sub-20hz frequencies. Not so good as a mid-bass driver. Definitely not as efficient as Polk claims - it's no more efficient than most other drivers in its class.
I'd say the best application would be some sort of compact dual-opposed design - takes advantage of the size and weight advantages, while addressing some of the trade-offs. I've got mine in a 3 cubic foot sealed box, it's a good versatile sub I simply can't think of any reason to complain. I'd liken it to those new Pioneer speakers at Best Buy. Faithful transducers of sound, a 'plastic fantastic' that uses nothing but tight-tolerance manufacturing techniques to achieve quality at a price point that was previously unimaginable. To be clear, the MSRP is a joke... but priced around $100-$150, I think it's considerably better than Alpine Type E, which I've had in my house at the same time as the MoMo, side-by-side.
The one thing I do not get - There keeps being mention of neodynium magents. It sure looks like there's a traditional ferrite magent on the motor. I'm confused.
I gotta say, I really, really tortured my MM1540 - ruggedness is exactly the quality I would ascribe to the MM1540. I've destroyed a number of Alpine, Kicker, Eminence and Polk subs (DXi) but short of plugging the MM1540 directly into a wall outlet or hitting it with a sledgehammer, I don't see how you one could hope for something more rugged. This is especially true for the lightweight basket and motor - you could mount these subs in an ATV used for offroad competition and expect them to hold up, mechanically.
Originally Posted by Ricci
It is 25mm xmax p2p.
Previously discussed here. http://www.avsforum.com/t/1431014/my-first-ever-diy-sub/90
Look for a picture of the back of the motor. It should tell you quite a bit. This is a driver for shallow mount apps with all of those associated trade offs. 2" voice coil but it has an LE rating of 5.1mH at 1khz which indicates no inductance rings. Power handling is rated at 425 watts (And this is a car audio driver). QES is 0.56. Total depth is less than 5". Not saying it can't be used well or sound good but shallow mount designs usually have to juggle quite a few things that end up impacting performance negatively.
At $130 you are getting into the DVC15, RS series, Alpine Type E, Obsidian 15, etc...Any of those would probably be more rugged, sound as good or better and have more displacement.