I finally had a chance to check out a system I had done a few years ago, my friend said that everything would be cool and then the bass would just lower in volume after awhile.

I checked over all the connections on the amp, voltage is good. Grounds are still solid. Tested for voltage present on RCA output.

I look at the deck and notice it is a Sony. Now the cake.
Apparently my friend had swapped out the old deck for the Sony, with a little twist and some black tape.

As I pulled out the deck, I could hear speakers fading in and out.

After close inspection, all the wires had been barely twisted/taped together and my favourite: the black wire was not used at all. I'm guessing that somehow the casing of the headunit was grounded out on some metal in the dash.

It was just enough to keep it going, but when it came time to deliver preout voltage, there wasn't enough juice left to do it.