amp problem?

i dont know what happend becuse yesterday my audio system was working fine and now it is all out of wack.  i went to get into my car last night and i heard a thud from my subs then nothing, so i thought the amp went. well i turned my car off and then back on and when the amp turned on all it did was make my subs hit in and out really fast. so today i hooked up another one of the same exact amps and it did the same thing?  here is what is weird about it.  the amp will play my subs normal up untill i try to turn the gain up. then it does the same thing and makes the subs rapidly hit.  when i turn the amp back down the subs are fine again.  i have no idea what is goin on.  if anyone has any clue what this could be please let me know becuse i have no idea what this could be.  the setup that is in my car right now is:

0 gauge from the battery to a 100 circuit breaker to a 2.5 farad capacitor then 0 gauge into my amp with a 0 gauge ground.  the amp is a ma audio hk401sx and it is running 4 w3v2's all with 12 gague wire run in a series at 2 ohms

anything else anyone needs to know just ask and please can someone tell me what is wrong?

i ran new rca's and it did the same thing from the sub out fron the hu and then i tryed a diffrent pre out so i hooked it up to the fron preouts and it works, so i guess i blew the sub pre out? i didnt even know you could do that, but how do i get that fixed becuse it is a preaty decent kenwood cd player it is a kdc-mp7028 that wasnt to cheap so i dont just wanna throw it away.