Audio Research - should I replace the valves?

Real NOS tubes last longer than the current Russian or Chinese production. If you want current production, maybe you could buy Sophia Electric KT88 big bottle. They are better sounding than any KT120.

should I replace the valves?

yes... as many other experienced hands here have said...

Probably a bit of a production from the UK, but when it is tube replacement time on my ARC amp, I ship it to them. They will give the whole thing a once over, test the tubes, and replace as necessary.  They are great to work with and don't keep it too long. And they set the bias. I realize it isn't a difficult thing to do at home, but I don't want to poke around 480V circuits unless absolutely necessary. 

Of course this means no fancy after-market tubes but just what ARC installs.

I can easily get the AR amps to Absolute Sounds, the importer in the Uk who will replace the valves but I’m told it takes a while and is expensive (anyone any experience of this?). Or I buy and replace/bias myself. But exactly what do I buy/for the Ref 110?

I've owned and steadily enjoyed a McIntosh MC275 tube amp and C2600 tube preamp since late 2017.  I've never tried to estimate the cumulative hrs. of listening, but it's up there.  I purchased a complete replacement set of replacement tubes for both components from McIntosh a couple of years ago, and stored them away for eventual replacement. I don't recall seeing any bias adjustments and frankly didn't know it was a thing.  I suppose it's time to swap them out!