about the only thing you can do without overpowering the subs power wise, is to put them in a bandpass box if you havnt already. they "sound" louder than a sealed box does. but they are not as clear and defined lows as a sealed box gives.
1991 318is aka: the torque-less wonder!
mods: TSW Hockenheims, H&R springs, Bilstein shocks, Supersprint exhaust, Jim Conforti chip, K&N, Z3 short shifter conversion, custom made strut tower brace
1988 Ford Ranger STX