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As I always advise in situations such as this,...

ask Ralph. Ralph knows his circuit better than any man alive, and he usually guesses correctly as to cause, even from long distance.

Meantime, you apparently have a volt meter. You don't mention what Atma-sphere amplifier you own, but it apparently uses 6AS7s, which means you've got a lot of 6AS7s up and running. Can you measure the plate voltage on each bank of output tubes? (There are separate supplies for each phase of the output stage. The supply for each phase is shared equally among all the output tubes on that half of the stage, so the plate voltage across one tube should be identical to that across any other, on that phase.) Having done that and if the plate voltages are equal on each phase, you could next measure the grid bias voltages; if one side is much more negative than the other, it indicates that that half of the amplifier is closer to shut-off than the other, not doing much work, which could account for that apparent low power without much distortion or any peculiar noise. The next question would be "why?"

Don't do any of this if you are not experienced working around high voltages and currents. Never touch the chassis with both hands at the same time; I keep one hand behind my back or in my pocket to remind me not to use it, when messing with a live circuit.

Edits: 09/25/15 09/25/15

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