I haven't had my gear long enough to be sure about the balance question but it sounds like bunk to me. A light bulb stays lit until it goes out. I personally plan to replace all the output tubes if one goes out because they went in at the same time. This would seem to limited any imbalance problems as well. If you stick with a known manufacturer who stocks the tubes you shouldn't have any problem finding them. I'm not sure on single ended tubes but my manual estimated 5000 hours or so on tubes. If you buy ARC or BAT which uses the Russian 6922, they are said to last about 10,000 hours. If that tube is the reason the new ARC gear sounds like it does, I'd avoid it like the plague.