The "wrong" tube can make the sound bright and not musical. Have you tried different tubes?
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A malfunctioning tube can cause all kinds of problems including sounding bright, loud popping and distortion. Try removing the top cover and lightly tapping the tubes while the system is on. Listen for any unusual sounds. If a tube has become hyper-microphonic you will definitely hear it. If you have spare tubes you can try different tubes. If you don't have spare tubes get some, it's part of owning tube gear.
Rrog is correct IMO. Use a pencil's eraser end, and try swapping the position of the tube if it uses multiples of the same type-ie: a gain stage will be more sensitive than a follower stage. Therefore an unacceptable tube in one spot may work (mostly) fine in another. If it is a 6922/6dj8 I would surely try this as they tend to be microphonic.
Yea, tubes seem to be able to go south in ever more innovative ways, and I don't believe there's any reliable rule of thumb. Echoing the above, tube performance can be awfully subjective, and trial and error seems the best way to go.
Way I see it, always helps to have a spare set lying around. Bottom line, one day you'll need them, might as well have them. Having them in the meantime, however, permits you to gather all types of useful info regarding when you might need them.
If you've got some offending bits coming out of one channel, the trick is to swaps tubes in pairs between the two channels (in the event you have more than one tube per channel). If the offending bits switch channels, that's your culprit. If you're not a fan of the sound coming from both channels, it's statistically less likely (but by no means impossible...) that you have a "bad" tube, and more likely that you just don't like the sound of the tubes you've got. Or, as 4est notes, could be -- if you have multiple, interchangable tubes per channel -- switching their position in the in the chain might help. As far as tubes, there are obviously way too many alternatives, if tube rolling is your thing, and you should be able to find something else that suits your taste.
If the whole shooting match seems bright, or flat or otherwise not so hot, this is when the extra set of tubes comes in handy -- and the only way I know of to really shed some light on what you're hearing. If monkeying with channels doesn't identify what ails you, swap the whole batch out: if it sounds the same, then tubes ain't your problem, put the old ones back and keep looking (knowing all the while that you've got your spares when you need them). If the clouds part and the sun rises on a better tomorrow, then it was your tubes and replacing them was obviously a good idea.
Best of luck.
Your perception is correct, trust your own ears. Some tubes sound too bright in some gear, but it is subjective. "tube rolling" is the quest for tubes which sound best to you in your gear. For example, in my GNSC-modified AR PH3-SE phono pre, modern electro-harmonix sounded too bright, NOS Amperex too mellow, Phillips and Telefunken about right. So far.