How far over the limit can I go

2000 hrs use recommendation is conservative by audio research

you can extend a good 20-25% past that if you are careful don’t listen loud or have had to drive speakers

the problem with arc amps of your ref110’s lineage is that there is no tube failure protection, so a failing power tube can (and often does) take out a bias resistor and maybe scorch a little section of the pcb, needing a minor repair

given the size and heft of the amp, even a minor repair becomes a pain for handle and ship

so most play it safe, replace tubes sooner than later, follow their recommendation

you can always monitor how close to being used up your tubes are by how much the bias current needs to be turned up from when they were new, i track that...

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.