i would suggest (for better SQ and SPL) that you stick with the same size subs (don't mix 12's and 15's)! and would also suggest you go with the 3 12's instead of the 15's for better SQ and i dont think you will have to worry about losing out on too much SPL with the 12's in a Tbird. i dont think there is enough room in a Tbird for 2 15's to be able to maximize their output potential. just my opinion, hope it helps!