I got both channels working again! I took apart the board and put it back together, and now both channels are outputting - the right channel is still quieter than the left, though, and as far as I can find from other threads that is a resistance problem? the right heatsink is still less warm than the left, haven't checked voltages yet but I did notice while reassembling that one of the insulating thingies between the transistor and the heatsink was ripped so that might be causing this? is it possible to get replacements for those?