i'd go the c3 route if you have the power for it.

there's nothing wrong about the m-class, i have a single 12. but the c3 is actually louder if thats what you are going for.

if you want low cost and loud, the powere reference would do the trick. in fact i think they (gwinnett sound) has the pr 10's in a slotted box for something like $300...