Originally Posted by mustangmike6677 View Post
The amp is a Alpine mrp-m450 mono block
the subs are audio tech 12s set up for 4 ohms apice series ckt
That may be your issue right there. If your running dual 4 ohm 12's in series, that would net you a total of 8 ohms. Your amp is rated for 400 watts at 2 ohms. Running them at 8 ohms would be giving them around 50 watts per sub. Try rewiring your subs in parallel. That will be a total of 2 ohms and give you your full 400 watts.
Also check your crossovers (LP Filter) to see if they are set correctly. Not set at max or anything. They should be set around 70-100 hz.
And by the head unit settings I mean that you have the sub output turned on and set correctly.
Heres a good site for reference: http://www.the12volt.com/caraudio/boxcalcs.asp#ser