Something to keep in mind for tube gear owners, is your tube Bias setting accurate?, no matter whether you have manual, auto or fixed bias (and the variations of) have your bias resistors drifted from spec values?, if so you bias settings are not going to be accurate. This generally applies to any gear that uses tubes for audio.
Best case scenario with resistor drift is that it might be just off a few points and no noticeable problems, worst case scenario is that the circuit will not bias the tube to design spec or even worse the circuit cannot bias the tube within in it’s safe operating parameters, oops there goes another expensive power tube !.
Pretty simple to check the resistors, if you are handy with a multimeter and own modern gear that typically discharges the caps on power down, if not sure you need a tech or have a tech background. Power down, unplug and let sit for an hour and compare the values from the resistor’s coding or markings to measured with the multimeter. If you are comfortable around high voltage, you can also measure again at operating temperature with power on.
Putting in high quality new tubes, NOS or new production tubes and getting issues where used tubes work fine can be an indication that your bias circuits may need attention, the resistance check is a good start.