Jc, might it be possible that the problem is stemming from upstream? Possibly at the amplification level?
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Swap your left & right speaker leads, and see if the problem stays in the same speaker or follows the speaker cables. If it follows the speaker cables, then it's upstream of the speakers. Next switch the left and right inputs on your amplifier, and see whether the problem stays where it is or switches channels. By following this approach, you can isolate which component the problem lies in. My first guess would be that you have a bad tube somewhere.