Weak whole channel could be any of a number of things.
One thing I would check first off, are the output tube screen resistor voltage dividers in the power supply. If the voltage on the screens of the output tubes is low, it definitely won't work right. There's big ceramic resistors near the output transformers (two pairs of resistors, one pair in front of the OPTs and one pair between the OPTs and the output tubes- one of each pair is for the left channel's tubes, the other for the right)- if the screen voltage isn't right on the suspect channel, it's time to check and compare the values of those resistors, to what they're listed as in the schematic.
Also, if the cap section in the can cap for the screen on that channel (there's separate screen supply caps sections for each channel, in the can cap nearest the rectifier tube) is partially shorted, it could also wreak havoc there. That amp looks to have never been recapped, so that's not impossible.
Check the bias too. If it's way too high or way too low,it will obviously cause issues... and each channel has its own bias pots and decouping (electrolytic) caps.
Also, how do you have the output terminals wired to the speakers? I usually like to start with both channel's negative speaker wires together on the "G" terminal, and the positives on the respective 4, 8 or 16 ohm taps. This takes the phase switch and the odd "damping control" wiring out of the picture. Less variables to start out with...
Regards,
Gordon.