Valve replacement - matching

[Edwin]

At present I don't need to face this problem, but the time may come. I have the WD88VA with four KT88 valves. If one of them failed, I assume I would have to replace it and its corresponding valve in the other channel with a matched pair. But that wouldn't account for the difference in match between the remaining two, and the one that has been taken out to allow for the new matched pair might be a better match with one of them. The ideal solution would, I suppose, be to replace all four with a "matched quad". You'll see that I'm no expert, and I've been reading all sorts of warnings about valves sold as "matched" pairs or quads that don't live up to the description. Any advice that I can put in storage for the future?