Run your HU with Sub System 1... its somewhere in setup. This way your subs volume is independent from the rest of the car, enabling you to basically have your speakers in the car at around 10 volume, or wherever it doesnt distort, and your sub to be louder. If that doesnt work, try just fading the volume towards the rear (if distorting speakers are in front).
Or just get new speakers, but if you just tweak some things here and there you can do whatever you want.