The only difference I can see between the Stage Lead and the Studio Lead is the power supply to the power amp and a couple resistor values there. The preamps appear to be identical and use the same board.
Isolate the problem. APply a test signal and trace it through the channel. The op amp stages are conventional. I donl;t know what the CMOS IC is at IC4, you tell me. But whatever it is, is there signal coming in and out at reasonable levels? If you changed it, it is possible the brands matters in this application.
And the TL604 up at IC 6 is always suspect, but it is just a switch IC, so when in the channel selected, does the signal look the same at pin 3 as it does at pin 7?