If I replace my 3 Bostons 10 inch's with a single Digital Design 10" and feed it enough power, it should sound better? I'm thinking not only from a loudness point of view but SQ. I also want to move the amps from the false floor to the sides to help heat buildup and therefore reduce the occurence of thermal cutout. I also want 2 do this 2 reclaim some boot space. Any thoughts/comments?

save ya money, or just get 2 12.5lf's

If you wanna clean up boot space then by all means, SQ should be better with less subs

Are the bostons Pro's or competitors?

And what kind of power do you have for the DD?

I don't think doing this will gain you much though...

Personally I'd go with a 9512 rather than a single 9510.

The depth of the sub is going to dictate the depth of the box, even making the box as shallow as possible and 750mm wide (typical WRX box) you'll easily have 1.75 cubes which is enough space to do a 9512 proud. Perhaps look at the new 9112, it's a new model with even more SQ refinements (but lower power handling).

Also depending on power, a 3512 might be an economical choice, it sounds 95% as good as a 9515 up to 1000Wrms.

As for amp cooling, 4 fans in push/pull configuration would keep things cool. Or change to a D class for the sub(s) and bridge one of the 4244s for the fronts, this will knock out a tonne of heat and make you front end come alive with the extra dynamics and head-room.

Phil