Have you tried all swapping new preamp tubes in instead of switching the old ones around?
Did the amp work fine before? Try the usual suspects; clean the preamp sockets with a quality contact cleaner, along with respected channel's volume/gain pots and switches...
It's hard to pinpoint the fault without tracing the signal...
There are a few things that could be at play but a bad solder joint in the signal path or a cracked resistor or bad capacitor could be causes. You could try injecting a signal and then lightly moving the parts with a non conductive tool to see if that brings back the sound thereby isolating the problem. I have experienced this issue before and a racked resistor was the culprit.
Transistor TR101 is open, when channel 1 and 2 are selected, to mute channnel 3. When ch 3 is selected, it must be closed (hi imedance). Try voltage on gate of TR 101, if it changes, while channels are changed.