Sirs
If you may allow me, Alpine 15" Type R can eat 2 Pioneer 308, any day on launch, no matter how good you tune them.
you need a really good power handling Signature series SUB @1000W or more to go more louder then 15" Type R, like W7, MTX 9500, Kicker L7, Alpine Type X.
Even single 15" Type R will be much cleaner, having deep bass and loudness then 2 308 or GT5 12.
The scene can only change if you position 2 12" Type R.
Very few people with some deep pockets and extra understanding, knows the limits of 15" Type R
Regards