... what exactly is 'critical'? What part does it play... 2000hrs seems a very very low figure for replacement - what do you experience, in SQ, after that time ...
The 6550 tube in ARC Ref5/Ref 6/Ref phono is in the power supply. It will sonically degrade after a few thousand hours, and can fail outright before 3,000 hours, ime. You really don't want that to happen. If the cost or frequency of replacing vacuum tubes is a great concern for you, it's likely that ARC Ref preamps aren't your best choice.