How far over the limit can I go

Probably good for another 1000 hours before noise sets in! You can go by the getter flash - when it starts to turn brown that is a sign of old age! 

Horseshoes and  hand grenades. Kinda sorta maybe.

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.