I would check the tubes to see if they have drifted a bit. Then I would check them maybe around a hundred hours.
I also own a Ref 110 but I don't use ARC tubes. ARC burns their tubes in a bit before they are matched. This tends to make the matches much better than tubes that are factory matched. The Ref 110 is an excellent amp. I have owned mine for a few years now and it never ceases to amaze me.