Weird issue with amp gain???

My neighbor sold me a 99 2D Accord with 2.3l V-tec. It already has a JVC KD-S29 head unit installed. When I put the gain about a quarter way up my subs start pushing in and out at a low frequency. When that happens my voltage drops to as low as 9 volts. There's also a faint hum coming from the subs if I switch to full range. I've narrowed out the amplifier since I have 10 other amps and they all done the same. Tried different ground areas for my head unit and no dice. Also setting the gain without any music playing causes the subs to fluctuate. I wouldn't think the amp would kill my electrical when there's no sound being produced. All I know is I am stumped!
 
It stopped raining and I had some daylight to work with. I did that little wire fix. From what I recall when I first took out the HU the remote wire was exposed. So it probably touched ground and burnt out the internal shield/ground fuse. Couldn't believe it was that simple! Thank you a ton for your help!