arent those subs rated at 600 RMS?? first of all i would make sure you have them in phase. if u dot want to take the subs out and make sure the wiring is correct, disconnect the wiring at the box take a 9 volt battery match up positive terminal with positive on the 9 volt then touch the 9 volt across the terminals speaker will either push out or **** in, do it with both sides make sure they go in same direction. Do you have an ampermeter? or a multimeter? whats the load you have it at 1 ohm?? so if you really have a 4k watt amp what you want to do is take your subs rms add them so 600+600= 1200. now you want to set your amp to match, burn a cd with a 60hz continuous tone, so we want 1200 watts rms.
so Voltage is = to the square root of power times resistance, which is 34.64V. so this is what your looking for, if you have an aftermarket cd player, safely you can probably turn it up to within 4 of the highest volume level so if your max level is 30, 26 is probly where i would go, turn all yer gains down, bass boost down, lowpass filter all the way up. with the cd player playing the tone turn gain up till it hits 34.5-36 volts you should be fine, now you can hook up your speakers again, turn your lowpass filter back down to what you had it set to. play the tone again measure the voltage again on the output of the amp if its holding at 34.5-36 volts your really doin 1200ish watts, and if your in phase and still sounds like ****, its your box, build one to spec. otherwise your as good as its gonna get. If your not holding around that voltage your electrical system cant handle it, add a battery. Hope this helps
