Thanks, I will try without the output transformer.I'm running out of ideas. I would also disconnect the output transformer and see if the naked amplifier also shows the behavior. A good idea would also be to trace the signal through the individual stages and see where the high frequency are lost. Be careful with your interface! HT!
Have you replaced the tubes?
How about an additional test with REW or RMAA?
I have replaced tubes, and I’ve also tried tracing the signal with a scope. Right at the input I see HF loss with the input transformer connected (very slight loss), then after each tube there is slightly more loss, so it’s just been very hard to trace where it is occurring.