Re: JBL or Velodyne
I have a Revel B15 (x2) and can report that if you can find one used at less than $1600, you're doing well, so I suggest it shouldn't be in this comparison.
Alternately, in my second system, I have 2 HSU's, which I still think are the best bang-for-buck subs out there, and if you call them, many times Dr. Hsu will deal with your problem directly over the phone. Note that Outlaw subs mentioned below are, in fact, rebranded HSU's.
Check out the HSU's, and try audiogon.com for good prices on used stuff.