I also think a single 12 might be a better replacement for the dual 10's.

Hmmm... I've never actually run 12's in my own car, so this may be just me displaying my ignorance. Take it for what it's worth.

In my experience, 12's (compared to 10's) come out sounding rather atonal -- no warmth, no musicality, just "blumph blumph blumph" bass sounds. This is what I've heard in other people's cars with 12's, and I can't vouch for how well the rest of their system was set up.

I have heard one car running 12's that was superb -- a $60,000 installation in a Lexus. He was running home-audio 12's at 8-ohms with tube amplifiers and to my certain knowledge at least three years worth of very serious tuning and tweaking, some of it by highly professional people with nationally-recognized reputations.

Theoretically, two 10's in a common enclosure (running mono, of course) would go lower than a 12, due to acoustic coupling. (The lowest frequency is primarily a function of cone area; two 10's = 157 square inches; one 12 = 113 square inches) and you also keep the advantage of the responsiveness and higher frequency capability of the 10's. In practice this advantage is small enough that there is disagreement about whether it really works.

I'd stick with the 10's... but that's just my opinion.

tanstaafl.