Hey-Hey!!!,
Do you have any way to get the tubes (the 6L6's) tested? Or perhaps swap in a known good quad?I would lean towards testing, in case there is something in-circuit which is dangerous to good tubes. Not just testing in a tester, perhaps another amp as well.
Since you have the schematic, does it have any voltages on it? Can you determine if it is misbehaving in an outwardlymeasureable fashion?
cheers,
Douglas