Here's the deal. I swapped in some Gold Lion repro. kt 66 tubes into the VTA ST70, as recommended by Bob and Junky Jan.
The amps been singing along nicely for months with many hours of play. Probably more than 50 hours already.
I pushed the tubes down gently in their sockets (power off), so they all looked... well, straighter. When I turned the amp on, the front left KT 66 was glowing brighter than all the others -- way brighter, and I was hearing popping from the left speaker. I checked the bias, and it was way out.
VOM read about .44 on both right side tubes, but on the left, I had something like 3.6 and .22.
I was able to re-bias, and the noise (and brightness) settled down.
Next day, same problem, but this time the front left potentiometer had no impact on the bias voltage at all.
I assume I've got a problem with the tube, so tonights plan is to move the tubes around and see if the problem follows.
If so, that'll tend to confirm it's the tube. If not...I guess I've got a problem inside, and better open it up and snoop around.
Alternately, I could go back to KT 88s I've got in the closet.
Suggestions and advice are eagerly welcome, and I hope this isn't a waste of bulletin board space.