Tube might be the problem.
If you can not check the tube, use the new matched new tube to try .
Pathos Heritage
I just got my Pathos Heritage for my home in the USA, but I am experiencing something I have never seen before; when I first plugged it in with the volume at half it sounded pretty low, kind of scary low for a 80Wpc amp running Avantgrade Duo Mezzo XD speakers or Tannoy Kensington.
After running it for about 100hrs the loudness dramatically increased.
I have owned and own quite a few amplifiers and never seen something like this before.
‘this is REW’s measurements at the first few hours
and here after about a hundred