Neither. Channels one and two on that amp share an input tube, using separate halves of V1 which is a 12AX7. It is unusual for just one triode of a 12AX7 to go bad, but it is possible and as I mentioned, it is easy to check. Just sub in a known working 12AX7 in V1 (look here for the tube layout).
While swapping tubes is quick and easy, I think it is more likely that it's going to come down to a mechanical or electronic failure somewhere between the input jack and the point where the two channels come together, which is the junction of R24 and C12.
What often helps at this stage is a very thorough visual inspection of the circuit in question. Look for a loose component lead, corroded contacts, burnt resistor, leaky capacitor, suspicious solder joint. Check the input jack really well especially it's connection to ground.
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