The very least you should go for SVETLANA Winged "C" 6550C rectifier.
Look for ones made before factory burned down. Ones made later is different and not very good.
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.