Why did this fix my problem?

I guess there was an impedance mismatch between your amps and your preamp. It sounds like either the amps were oscillating, i.e. motor boating, or the mismatch was forcing the preamp to oscillate pulsing the amps . That's the reason why the bias would fluctuate the way it did. It good you solved the problem because issues like that can cause output tubes to arc and woofers and tweeters to seize, depending on the oscillation frequency.

Hifigeek1, The woofers oscillating is where I saw the problem in the first place. I thought it was DC leakage at first. I did lose 1 kt77 tube. I tried everything I could think of..swapped tubes in amp and preamp AND Modwright Logitech transport, cables, even brought out an old NAD integrated that I could use the preamp section to run the amps which is when I finally got it to settle down.

What I don't get is what did lowering the impedance do to make them compatible?

I assume that the interconnections between the two components were via unbalanced (RCA) cables, since the preamp does not provide balanced outputs. I see that the amp has both unbalanced and pseudo-balanced inputs. I presume that there were no jumpers or other connections on the XLR inputs.

This is just a shot in the dark, for which I can't formulate a precise explanation, but perhaps the problem would not have occurred if the amp's inverting input on XLR pin 3 had been grounded (e.g., with a jumper connecting that pin to XLR pin 1).

With the original 100K input impedance, the unconnected inverting XLR input would have more closely approached a "floating" condition, compared to when the input impedance was subsequently changed to 47K. That would perhaps have made it more likely that the amp would behave in an uncontrolled manner. Without detailed design information on the amp it is probably not possible to say what that behavior might have been, but an oscillation of some sort seems conceivable to me.

The different (and probably lower) output impedance of the NAD that you temporarily connected would have changed (and probably reduced) the impedance between the non-inverting input signal path of the amp and ground. Again, without having specific knowledge of the design of the amp's input circuit it's hard to explain precisely how that might have prevented the oscillatory condition, but it seems conceivable to me that the changed circuit conditions might have had that result.

Regards,
-- Al

Al, Thanks for chiming in! I have read many of your posts over the years and your thoughts are always insightful. My amp doesn't have any xlr inputs though. I am assuming you read the review on the Crescendos in Stereo Times, I have read that also. There aren't any extra holes so this pair never had xlr inputs. These are only identified as TPA-60, not Crescendo. The amps are a push/pull design and Direct Coupled. That is about as much info as I have.