I would recheck the wiring.
So I have an old Punch 250 a2 that I hooked up last night. When I turn on my Kenwood h/u the amp turns on fine. For some odd reason though, if I plug the rca into the right channel and then plug the sub in, I get absolutely no sound from the sub. When i try plugging the rca and sub into the left channel the amp shuts off. If I pull the speaker wire out of the left channel the amp then turns back on.
Last year I had this amp and sub running in my Explorer without any problems.