Velodyne and JL Audio make some of the finest subwoofer speakers in the world. Unfortunately they do not represent a great value dollar for dollar in my opinion. They perform well, but there's definitely better to be had.
Give or take, you're trying to fill somewhere from 3000 to 5000 cu ft. That's reasonably big, and so I agree that 15" woofers or larger is the way to go, and arguably a resonant alignment will likely much more efficiently give you a good bit more output in the region from 15hz to 30hz (although many,
many, sealed is the way to go if you want performance down to |7hz).
Unfortunately what you pay for those is many times what you'll pay for an internet direct subwoofer. For the same price, you can probably get four internet direct subwoofers which will perform better. For example the Seaton Submersive and Terraform:
http://www.seaton-sound-forum.com/post?id=3364736
The Funkywaves 18.0 and 18.3:
http://funkywaves.net/catalog.cfm?item=fw_18_3
More than anything, having four subwoofers instead of two is well worth it, and the above options are just a few that will outpeform on a 1 to 1 basis, the JL and Velodyne offerings.
it`s not that JL and Velodyne are poor companies at all, just that their products are pricy and targeted towards people who need `Small and hidden` rather than the best performer. The 18.3 above, for example, uses arguably the best bass driver in the world (TC LMS5400 - an 18" driver which employs well placed demodulating rings and an advanced motor to maintain the same performance at full stroke that it does on light strokes) and a pair of passive radiators for a very efficient, deep, high output bass. The passive radiators are tuned to couple well with most rooms and should give flat in-room response (which will mean very tight perception of deep bass) especially if you have multiples (which really smooths out in-room response in the audible region).
I recommend at least 3 subs, and I strongly recommend going internet direct because the performance is arguably better, while the price is lower. The size, however, is not smaller, because internet direct companies don't need to sell you a tiny shoebox (a big compromise when it comes to subwoofers).
