Your bias supply from the secondary should be absolutely fine and there is very little history of any significant failure modes in this circuit of the MTS amps. It could be prone to drift if the pot was damaged or possibly made contact with the insulating bushing on the chassis AND the amp was being excessively vibrated by a speaker. Keep in mind most amps have their bias adjust mounted internally using a wheel type pot and there is no real potential for accidental adjustment.
That said, current production tubes are notoriously hit or miss. My money is on poor tubes. You are likely only aware of it because it's so easy to monitor on your MTS amps. For the record, I work on plenty of amps that sound and feel fantastic where the tubes aren't matched closer than 4-5mA apart or the crossover notch looks bad on a scope. Don't be afraid of it and use your ears in addition to your meter. They are your best tool.
Rob
