How far over the limit can I go

One of the best mechanics I ever knew growing up back in the day, a real experienced old guy, said if it ain't broke don't fix it. No wait, that's the cliche. What he said was, everything you do costs time and money and entails some kind of risk. No wait that's not right. Tell you the truth, don't remember exactly what he said. But whatever it was if it ain't broke don't fix it will do.

You got good expert advice about ARC amplifier potential damage when a tube does fail. Sometimes conventional wisdom ain’t. 

to @smrex13’s point

if you do swap in a fresh set of power tubes to hear them, BE SURE to turn down the bias current substantially BEFORE installing the fresh tubes, then, over 30-60 min playing music, gradually increase the bias to get to the 65 ma reading across the test resistor for each side holding steady... if your present set of well used tubes are properly biased now, that bias setting will be WAY TOO HIGH for the new set

detailed instructions on biasing (using a simple multi meter with hook probes) are in your manual

you don’t want to fry your new set or age them prematurely by doing this swap incorrectly

Also usually you can go ten over. But not in Virginia.

I can tell you that catastrophic tube failure can be quite expensive. It cost me $650 and the builder was generous with me because it was not that old. But I bought it used and thus no warranty. I also learned the pitfalls of buying an amp that  has not been broken in yet. Break in can happen either for the good or it  really BREAKS in,  IE failure. BTW It had protection too. But, it still took out a power supply. 

FWIW if you have a catastrophic failure, you will need tubes AND repair. 8 tubes should not cost $1k. Driver tubes  may be alright  since they have about double the life of power tubes. Just a thought.