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The set could be damaging the 6W6, or it could have just been a problem tube that got worse from use. I'd look for voltages out of speck on that tube. One thing to beware of is autobias on the tube (don't know if it is used in your set or stacked B+ sets, but suspect it is). If it is off the tube could be conducting too hard and get worn out prematurely, or conduct too little and have other issues.
Autobias basically uses a lowish value cathode resistor to 'ground' (in your set its a lower B+ rail) and connects the grid to the 'ground' through a high value resistor. The resistor is chosen so that if a specific current flows through it it will create a voltage difference between cathode and the grid that makes that current flow....Basically it's a feedback loop with all static values dependent on the cathode resistor value (assuming no defects elsewhere).
One thing comes to mind assuming a modulated channel with no sound or significant noise if the 'sound control' is volume and drives the grid of the output, but is isolated from that grid with a cap the cap may have failed and the control may be skewing the no sound bias of that grid outside of the designed range.
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