Noise only when Engine is running (Buzzing sound - increase with gain boost)

NorthStyle

A few things:

1) If the buzzing/noise (interference) was there before, adding another amp just amplified it (no pun intended).

2) You can't ground the audio outputs, each speaker has a +/- that needs to be connected to complete the series. You removed it, didn't complete the series, and thus lost sound (not sure how that would work if you grounded both the input on the speaker side and the output on the headunit/amp side, but never been tempted to find out). That being said: I'd replace them with good quality RCAs from the headunit to the amps.

3) Check your grounds and their distance. I've always been taught that grounds should be as short as possible. Where are your amps located? If they're somewhere in the trunk, and you're running your ground to a seat, that's a pretty long distance. Also, I'd ground each amp to a different point.

4) If you're going to run your amps off of the same cable, use a power distribution block to separate them. For example: I only have a single amp, but my compressor and crossover require power as well; so I have a 0 gauge (fused, forgot the size) from the battery to the distro block, then a 4 gauge (fused) to the amp; and two 8/10 gauge cables (each, fused) to the compressor and the crossover. Each has a different ground. The only thing that is shared is the remote/signal wire from the headunit to the amp which then feeds the crossover.

My advice would be to trace the initial source of interference. I'm sure BigAudioFanat or RonRizz will chime in later as both of them are leagues ahead of me knowledge-wise.