The most important consideration for getting excellent bass response and performance from your system is to make absolutely sure that your sub is both placed properly in the room - in relation to your seating positions - and that it is both integrated correctly at the critical crossover reigion (speakers/sub relationship @ seating location). All that, and that the sub is EQ'd out for any issues within your room/setup. The vast majority of people setting up speaker systems fail to properly address and integrate the sub and speakers into the room, in relation to good seating positions, at the expense of good solid, high fidelity performance from the system!
I have sold just about every 15" and 18" "super sub" out there, and have heard most of em installed improperly as well...yeilding less than stelar results, accordingly.
I have also seen very expensive subwoofer setups in multiples, where they canceled each other out at their respective lower frequency ranges, due to phase cancelation issues. You sit closer to one sub than the other in a given seating arrangement, and you have combfiltering problems, and weak bass, holes, etc. Of course, let's not forget eq'ing out the huge humps, and avoiding acoustical holes in the bass response, as your typical rookie mistakes to fall into. Flat accurate, well integrated and coherent, impactful, fast bass response is the goal, for certain.
You simply MUST take care, and learn to set the subs up properly, measure response,take into consideration seating in relation to speaker/sub placement, etc, if you wish to get great performance from your bass! If you don't, you might as well get lesser equipment, and just learn to set it up well, cause you're wasting your money!! That's my experience.
I once helped a guy set up a system, where he had a large $5k M&K MX5000 and large 15" Earthquake subwoofer set up in the system. From most of the seating locations, the subs were set up so they were out of phase with each other, and canceling out all the bass impact! Very common, and easy to fall into this problem, without knowing what you're doing.
When people tell me they're using multiple subs, my doubts immediately kick in on how well their system will actually play out. More often than not, they don't know they have issues, and start making assumption about the quality of the pieces, rather than actual USER ERROR that's really the culprit.
I think most people really would do better in trying to integrate one well placed sub, and getting an auto EQ (Audyssey, etc) into the equation. Then they'll likely end up with better results, more often than not. FYI
In response to your question, you may just find you'll get better results - if by accident, if nothing else - with two JL subs, than perhaps the 15" Velo. Then again, you could very well likely have nowhere near as good results, depending on how lucky you get, or how well you end up integrating the multiples into the system.
Just don't fall into the trap that many who post reponses on this forum, regarding their experience with gear in their system/setups, on how good or bad some product they tried actually is! There's often user error that's not being considered, is all I'm sayin. In the end, all you can do is try for yourself and see what results you get.
Great thing about the used market is that you can trade with low risk..even sell what you don't end up using.
Good luck
