Like pucho mentioned, you can narrow down the testing a lot if you use the line input instead of mic, and turn all the buttons off on the front (so they are not lit up.) This will remove a bunch of things from the signal path: equalizer, compressor, input transformer, Hi-Z circuit, half the gain knob, tubes V1 and V4, some relays, etc. Much simpler signal path. Plus the VU meter will show output signal strength instead of compression. But if you can send it to him, I'm sure it will get figured out.