You should try to match your subs to your amps. I mean power rating wise. If your subs are bridged, they can usually handle twice the power. Your 600 amp sub is just not driving those speakers. Speakers with heavier construction, etc, and you are under powering them. Having one 10" sub in a decent ported (tuned) or sealed enclosure is going to hit harder than 2 12" subs barely driven to give you sound.