That should work great for you, the 10W3 dual 2 wired in series.

But - if it was me, I like to screw with stuff and see what I can get away with. I'd try the dual 6 and throw a 3 ohm load to the amp. MTX is known to be pretty rugged, and I'm willing to bet it would take it. Of course if it didn't, you'd be forced into a new speaker or amp. You could contact MTX directly and ask... Historically they're good about replying.

You're looking for space apparently, so perhaps a single 12" is out. But if you're used to getting a lot of volume out of the 2-10's, you may be wishing to at least have a 12" single. I've had JL in the past, and I also really liked them. Stick with what you know I guess. To throw another option - I currently have a pair of Boston Acoustics 10" Pro's in my spare tire well, they only need .5 ft^3 each, so that could be an option for a sub too.

I do like the 10W3 dual 2 JL though, already in a box and everything. There's absolutely no reason not to get it that I can see.