Amps

Your amp is at risk of overheating and blowing if the specs are exceeded. Run bridged mode and get 2nd amp.

To get the most reliable work out of your amp, the speaker loads need to be the lowest impedance recommended. Make sure the 12v supply remains stable, good ground, larger wire (8AWG or better)will reduce voltage drop and a large cap(100,000uF) across 12v line will help with large current pulses. Keep the amp cool. No small air tight enclosures. Fans??
Tech stuff
E Squared/R=P. E(RMS speaker voltage), R(speaker impedance), P(watts). Only way to get more watts is to increase E or reduce R.