Just to make sure we isolated the problem, if you disconnect the 6 channel board and insert a signal into the Tape In inputs (use your iPhone, iPod, other device), the amp will work just fine?
Check the solder connections on the 6 channel board. Check to make sure the XLRs are not damaged. I would pull the 4558 ICs but leave channel 1 in place. Do you still have the hum? Test it with a mic or some other source. Then add ICs to get channel 2 working. Again, same thing. See if you can isolate which channel is the problem.