Plz help my Pc12-Ultra started buzzing

Hello there all.
I bought this sub used about 6 months ago and its been working great until yesterday. I had been tweaking crossover settings on my Yamaha amp and noticed a few crunchy popping noises during playback, then all of a sudden it started humming/buzzing fairly loudly. It only hums when plugged into RCA. Im really hoping my av reciever is dead and not my sub amp. Before this problem, if I turned the sub on before the reciever it would hum, but as soon as i turned on the reciever it would be reduced greatly: inaudible during playback. Now the humming gets louder when i turn on the amp(reciever). Also when I switch the crossover on sub to Disabled the hum changes to higher frequency. Pease help

Ok, What a crazy day, after some troubleshooting the problem seems to have resolved itself... here's the story.
I talked to a svs tech and we seemed to have eliminated everything but the sub cable. So I exchanged with a shorter rca cable (2 feet) I noticed no buzzing and told the tech thanks then after i hung up i noticed the power on my sub was off...(had turned it off to exchange rca cables) i prayed then turned it on. No more buzz yay.
But now im using the longer cable and its still not buzzing. So maybe something in the cable was loose.

Also I almost destroyed my sub in the troubleshooting process by running the sub from a random rca out on amp. The gain was halfway up on sub holy crap it was loud! I bottomed it out hard, was so scary. Also after lowering the volume i could hear frequencies from the hundreds to i think 1khz!!(crossover on sub was disabled) So during the bottom out it got unfiltered full range signal jeez. anyone know if these old bash amps have limiter protection?
Sounds good now though. I think I dodged a bullet today. But somewhere in my mind I know I should start saving for the pc-13 ultra.

Thanks all