Ok, I was right about the high level input being the issue. I did a test with my phone and RCAs and it was unbelievable how much more bass was coming out, I almost blew it up. So this is jerry rigging but not in a harmful way. I took a 3.5mm splitter and plugged the tape player in my HU into the splitter along with the RCAs going the amp and now it's all playing together. I have the bass nob to control the sub also. This will solve my problem until I get a new HU, but I appreciate you guys trying to help. thanks!