Your unit tracks tube life? About 2000 on the 6550 and 4000 on the 6H30P. Why on earth that valve, duno.. They're expensive, can't use any other valves either. KT88 are not recommended either. 6550s, and 6H30P only.
Regards
Replacement Tubes for ARC Reference 6
I realize that the tube replacement for ARC's Ref 6 are good quality, and when I get these from ARC they would be matched. I believe these are all Russian--good, but not great. Is there any reason to check for better quality tubes when I do a replacement? These are 6H30P and 6550WE tubes.