This was taken from the I/50 owners manual.
It is recommended that you replace the vacuum tubes of your I/50 in sets. All of the tubes in your amplifier have been matched to have similar operating characteristics, to provide the best sound quality and reliability. 6550WE tubes should be replaced at approximately 2000 hours. 6922 tubes should be replaced before 4000 hours. Upon replacing a power tube or tube set, the I/50 must be biased by a trained technician to ensure proper operating values.
It seems like the amp can’t be biased by the owner of the amp. If that is true that would definitely be a dealbreaker for me. Just sayin’.