Did you swap speaker sides to see if it follows the one speaker or if it is the room?
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Switch the small tubes, channel to channel, in your pre-amp. See if the volume reduction changes speakers.
If that doesn't make a difference, and especially if you have a critically aligned and triangulated set up, check your listening chair'a position to be sure some one hasn't moved it a few inches (I have to recheck mine every time the housekeepers leave). :-)