Clean the volume and balance pots and switch contacts? Do not use Caig Gold on plastic pots, it leaves a film. Just use cleaner, or Caig pot cleaner. Spray inside, work it, rinse, work it, repeat a few times. I use RS contact cleaner spray.
Check B+ and inspect PS caps? Small storage caps, just a couple 100uF electrolytics, but inspect those for burning or leaking. V7 is line stage B+ regulator, a 12BH7 tube making 402VDC. It has 640V on the plate. Have you replaced that? This would affect both channels tho.
Cathode resistor failing? Look at R23 and R28 and R25 on the failing channel.
R23 = 1.8k wirewound
R28 = 6.98k
R25 = 1.5k
If one drifts higher it can lower the volume.
They can be tested in circuit with preamp OFF, but verify storage caps empty (wait half hour after shutdown, then verify no DC voltage across R31) before testing resistors. I would not advise testing the B+ voltage yourself due to lethal high voltage when preamp is powered on. Since it is only one channel, the PS is probably OK.
Here is the amplifier schematic, in the pictures section:
http://www.arcdb.ws/SP8/SP8.htmlDisclaimer: Don't believe anything you read on the internet!