Unfortunately, you can also have a tube that is definitely "bad" without it being all that ostentatious about it. After a while, they simply lose steam and sound flatter and more constricted -- it may not make noise or advertise the fact that it's on its way out to pasture, it just won't perform. (Or, you can have the noisy, drama queen tubes that bitch and hiss and spit and moan on their way out, which is nice only because it makes them a whole lot easier to single out and shoot). If you come into a full tubed setup of unidentifiable though likely considerable age, I would immediately assume that the tubes are suspect. If you plug it in and it works fine, without any extraneous pops or overbearing tube rush or the like, then hey, it works. Whether or not the tubes are really giving you all that the gear is capable of, however, is likely unknowable without some context to compare them to. Id say, if it works and makes you happy, let it. Full stop. If it immediately sounds flat, constricted, thin, anemic or something like that, then Id suspect the tubes. Eventually, youll have to get some new glass for it, and the only way I know to see whether replacement tubes sound better (and, deductively, to guess whether the old ones were running out of gas) is to try it. Its sometimes nice to have a replacement set on hand, just in case. If thats practical at all, it might be worth doing sooner rather than later. Get used to the sound of the old ones for a bit (assuming theres no immediately evident problem) and then switch out the full set for the spares. If you have a preference, go with it. If not, put the originals back in and rest assured that youve got reliable backup when you need it. If you notice that there are some things that are better and some that are worse depending on what tubes are in the system and begin to rolling more and more tubes in search of the prefect balance, then youre in trouble and I wish you luck.