WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
Look at the schematic. That "beast" uses "Cheap Charlie" DC heating of preamp tubes. Preamp tube heaters are be used in the O/P tube bias networks. While that sort of thing is common in "vintage" units, it's dreadful.
Build a proper, regulated, DC supply for the preamp tubes. Separate RC cathode bias networks for each channel are also indicated.
Talk to Jim McShane about tubes, parts, and suggestions. He offers honest advice and fair prices.
I find myself behaving like a "stuck" phonograph record, with amps from this time frame. Replace the source selector switch. Hard wire a single set of I/P jacks straight into the low level mag. preamp circuitry, which gets set up exclusively for RIAA. Switches in mV. signal level circuitry are bad and tape head is passe, in this day and age. Make all the other I/Ps line level. Just under $5 buys a suitable, Lorlin brand, non-shorting, rotary switch from Mouser.