The NOS TungSol ( Harrison NJ ) 6550 is killer in the Ref5se, Andy at Vintage Tube services is good source.
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.