Audio Research - should I replace the valves?

Replace older still operating tubes and use them as spares. 

i don’t think they sounds as good as they did but no pops or whistles or any other strange noises etc - just a bit flat sounding.

This is a good cause to replace. Problem with too many tubes is you cannot tell for sure which one or ones reached end of life.

Normally power tubes wear out faster, a starting point to order a new set from ARC, with preamp ones to follow.

Thanks all. Not sure whether biasing the valves is beyond my skills but will have a read. Stupid question but assume you fit them and then do the bias? (Told you I might not be up to the job!)

Yes, you seat the tubes and then bias. As a long term ARC owner, trust me please and invest in a decent Fluke. As in Fluke meter. And don't sweat it. Except that finding a reliable source of tubes particularly for your amp is not so easy anymore. 

Tube replacement is simple. Do not touch the tubes with your fingers… use a clean cloth. Biasing is easy… here is a copy of the manual in case you don’t know where it is.

Do this once and you’ll be a pro. It sounds much harder than is.

Fluke meters are good and come in vastly different cost ranges. You do not need to spend a lot. I always have a multimeter around, there are hundreds of brands. 

https://audioresearch.com/wp-content/uploads/2021/06/REF110_Manual.pdf