Welcome to the place.
The two preamps are exactly the same with the exception of the final 2 transistor boost circuit on the PC203 board, so you can compare voltage readings etc. from channel to channel.
If you are sure that the new transistors are installed correctly, then check the voltage readings compared to the schematic or the other board.
Input a signal into the front panel jack and then trace the signal with your scope through each stage and see where the signal is and isn't.
One thing with the boost circuit switch, is that it is just like a bypass switch on an FX box. If the contacts are really dirty it can completely kill the signal coming out of the preamp.