There is something wrong with your amp.Lets start with Class A.In this mode only the 2 outer tubes are functioning,the switch lifts the cathodes of the inner tubes,taking them out of operation.When you are in class A socket 1 & 4 should be identical electronically i.e. same voltage, same bias current.When you put tube 4 in position 1 it is redplating because a malfunction in socket 4 has cooked it.The fact that tube 1 in position 4 redplates,where it didnt in position 1 tells me something is wrong with socket #4. The problem only seems to be arising in Class A mode,because to achieve Class A the current in the tubes has to be raised higher than it is in class AB,which is how the tubes are biased in Simul,although you dont see redplating in Simul whatever damage is in the amp is slowly taking its toll in this mode as well,it just isnt as severe as class A because of the lower current,but believe me,it is taking a toll.You seem to want to believe that the problem is solely tube related,it isnt.Putting a new set of tubes in is going to result in the same problem after time.Like I said earlier I dont know what the tech did that would call for a 3amp fuse in that amp,and the only conclusion I can come to on that subject is that he is ignoring the problem and forestalling an eventual failure.From what I read here,I dont think you are qualified to open the amp and probe any deeper,if I am wrong about your technical knowledge I apologize,but I would advise you to take the amp to someone who can.You need to find out why the amp was blowing the properly rated fuse (2.5amp),and fix that as well as any other damage caused by running the amp as you have been.If you ignore it and put new tubes in that amp they will eventually die,no doubt about it,and most likely kill the amp sooner or later.