I agree with what has been posted in this thread, more subs is better, a lot better. I am currently running three subs as that is what fits my 10,000 cubic foot room the best. Each new sub added very noticeable improvement in smoothing things out, eliminating boomy spots, and making the bass very musical. I also do not feel that in my case I need a 4th sub as my two main speakers are the very large Byston Model T signatures which each has triple 8 inch woofers, so the main speakers alone have 6 woofer drivers. I installed an Axiom Audio EP800V4 at the left side position which is their monster dual 12 inch driver sub which stands around 48 inches tall, to the rear I installed an Axiom Audio EP500v4 which is basically a single 12 inch driver version of the EP800, and to the right side I put in a Bryston Model T sub which is a triple 8 inch driver, 48 inch tall sub; the same exact drivers as used in the Model T main speakers. All of the subs are perfectly matched as Axiom Audio builds all of Brystons speakers and subs, even the amps are the same.
Axiom V4 sub amps all use the fully adjustable phasing controls which are a must. Once I have my subs placed in the sweet spot of where they work best for the room, I get down on the floor next to the sub and slowly rotate the phasing control until I hear the best in phase presentation of the music content. Its very evident, the bass signals just stop fighting each other and suddenly become one harmonious signal with maximum smoothed amplitude. And it doesn't matter where you are in the room, the bass is the same. It only takes 30 seconds to phase the speaker and once set, you just forget about it.
OP, if you have not purchased any speakers as of this moment, I'd suggest that you look at Axiom Audio and for your room size I would consider their EP500. It's a little larger than your 10 inch JL's, but its a very musical sub. If you can afford them, the Bryston Model T subs are fabulous with triple 8 inch drivers and I have this one crossed so that it covers the upper most range of bass into the very low end of mid's; they are expensive! Like around $6K each.
But if you have a budget of $3500, I'd get three EP500's and place one on each side of the room and one to the rear. Then phase them for the room and you will have some fabulous lower end music which will not be muddy at all! Another important area to keep in your planning is the amps in the subs themselves. Most all of the amps out there are class D amps with switching power supplies. The switching supplies have very little capacitive reserve so if you are playing content with a lot of bass, they will top out easily and not give you what you want. If you have a class D amp with a large capacity Linear power supply with a lot of capacitance, there is a ton of head room to let the drivers hit their capabilities without running out of steam; that was another reason I went with Axiom subs, they have massive linear power supplies (you'd expect that with anything coming from Bryston). If you end up with just one sub, you will have a muddy swamp no matter how much you try to move the thing around and play with EQ or whatever else. It just won't work out well. I might also add that the real wood veneer Black Ash is stunning! It's a nice satin raised grain finish which is just gorgeous. My two main Bryston Model T's are in satin Santose Rosewood which too is the most beautiful speaker I have seen; but the Black Ash is just as nice in a different way.
