Calvin and the manager of service recommended changing the 6550 in my REF5 every 2000hrs, if you wait until it fails, expect a large repair bill.
Also, having owned several tube gear in the past, I recommend paying the extra and getting them from ARC. They test each tube and warranty them. I also recommend always having a spare set of tubes on hand just in case. Also, they are now using 6550EH instead of C tubes because they think they sound better. When I have my REF5 upgraded to SE in May-June, I will have them retube including the EH tube. Happy Listening!
Also, having owned several tube gear in the past, I recommend paying the extra and getting them from ARC. They test each tube and warranty them. I also recommend always having a spare set of tubes on hand just in case. Also, they are now using 6550EH instead of C tubes because they think they sound better. When I have my REF5 upgraded to SE in May-June, I will have them retube including the EH tube. Happy Listening!