You don't have to set an amp to full power - simply adjust the gain to give whatever power your speakers can handle. The USX-4065 would be more than adequate for your needs.
The US Acoustics amps have adjustable hi-pass settings that will give you much much more control over the roll-off frequencies than a simple bass blocker would. Start with it set for 90 Hz and see how that sounds. If the 4" jobs still break up then add a 150 bass blocker just for them.
1999 Buick Park Ave Ultra
Head Unit: JVC KWHDR720
EQ: Audiocontrol EQL
Front Stage: Hybrid Audio Technologies Imagine 5.25 mids, Morel Maximo tweets
Sub Stage: Boston G3 10", 0.5 cu ft sealed
Amp: PPI Phantom 900.4
Rear Fill: Boston SE953 6X9's