Can this be caused by a bad ground?

[One Way]

Sorry, this is not something that I could make for you. The VERY first time I encountered this problem was back in 1992 when Linear Power introduced their very first four channel amplifiers. Nothing we tried would resolve the problem, but those types of cables didn't exist back then. They redesigned the input section of the amp and that resolved it. You might scour eBay for a pair of these and get lucky.

You also may be able to solve the problem by using an isolation transformer between the output of the deck and input of the amplifier. Something like THIS.

That's around what I paid for the amp lol... guess I'm SOL on this one. Thanks for all the help Tony, I appreciate it.