i lurked on the diy bass list for almost two years, determined to get the best bass for the lowest price - lowest hz, lowest distortion, highest spl's, most musical. (this was for music; i don't watch movies.) i also went & listened to some commercial offerings. vmps, velodyne, m&k. i ended up buying a pair of vmps larger subwoofers. at the end of the day, i had determined that, even if i went diy, i couldn't approach the overall sound quality of subs i got, for anywhere near the price.
this was ten years ago. i believe the same is true today, tho some of the kits may have gotten closer, for the $$$... and, vmps also offers kits, which means you can get even *more* bang-for-buck. if the vmps larger is too big, then check out their original & smaller. also, i recommend getting two, vs one, for music. if price/size is a consideration, i would get two originals, instead of one larger, for example....
another reason why vmps is better, imo, is if you go passive, you will get far better overall sound w/outboard active x-overs, which will be far more transparent than the x-over electronics installed in most powered subs. of course, many powered subs *do* offer decent electronics, like those in the top line velodynes, rels, etc. but, at ~four times the overall system costs...
doug s.