I think part of the issue as you have noticed might be because it is a class A amp and they are designed to handle the excess heat and once they reach a certain temperature they really start to open up. I have a pair of Cary 211 AE amps soon to be FE. They play in class A up to about 75 watts which is more than sufficient to drive my Tekton Design Ulfberhts which are 98 per cent efficient. The amps sound very good from the start but after about an hour and a half of listening when they get hot , they reach a playback level that is simply wonderful and similar to what you are experiencing with your Pass Labs amps.
I also have a pair of Odyssey Audio Kismet solid state amps and they barely get warm even after several hours of playing but they sound the same from the time they are turned on to when they are turned off.
If I have company coming over for a listening session , I will start the Cary amps about an hour earlier to get them at least warm. The downside to this is tube usage but the upside once they get hot is more than worth it.
So in this long winded response is that depending upon the tube amp, you may experience the same issue with them.
I also have a pair of Odyssey Audio Kismet solid state amps and they barely get warm even after several hours of playing but they sound the same from the time they are turned on to when they are turned off.
If I have company coming over for a listening session , I will start the Cary amps about an hour earlier to get them at least warm. The downside to this is tube usage but the upside once they get hot is more than worth it.
So in this long winded response is that depending upon the tube amp, you may experience the same issue with them.