The bias light may flash as you bias the other tubes. Just bias them once a s if needed adjust.
I know you are discussing an amp, but I recently had this exact problem with my tube preamp, especially the awful thump. Like you, put in new tubes and same thing happened. Was about to send my unit for repair when I decided to ask the distributer if there was a repair facility they recommend.
The distributer said before sending it in that the problem may be caused by a loose tube pin socket and there are two ways to check this; One is to gently pinch the socket sleeve using small pliers to make the pin contact tighter. The second is also quite effective but easier to do. Simply bend each tube pin slightly inward and outward for better contact with tube pins slightly bent in opposite direction. The point is to create more tension inside the socket holes for a firmer grip.
I couldn't believe it worked like a charm. Used the second method which took all of minute.
As a side note, it is remarkable how much more robust my old school preamp tubes including Telefunken, RCA and GE are built compared to newer Psvane tubes, especially the pins. I was not careful and one of the Psvane pins broke off, no chance of it happening with the NOS tubes.