It is usually best to replace tubes, in the units, they are being used. ie: pairs, or quads. It is a push-pull circuit, so replacing the pair, would be best. Tube matching, in power tubes, is important, because of the nature of push-pull designs. If you replace one, which doesn't match, it will unbalance the circuit.
How are tubes to be purchased in general?
I have never had a tube power amp. Although I have a tube preamp (AI3) it is relatively unfussy with tubes. If a power amp has a matched quad set of tubes, how do you replace them if only one goes bad? Most are sold in matched pairs. How many do I have to keep around for emergencies? Do all four ever go bad at once, or can I just pop in one or two. I talked to the manufacturer (Rogue) but the rosy picture of tubes lasting years finds me a little bit skeptical. Of course if this question is like an owner of a gas guzzler car wondering if he can afford the gas (tube replacements) please tell me so or to go get a SS amp. It's just that a cheap pair of matched KT88's/6550 go for about $70.00 new, two monoblocks mean 8 tubes so that is $280.00 investment.