Absolutely, positively sounds like classic "tube rush." Tubes do this -- all of them, to one extent or another. If it's so loud that it's noticeable from the listening position and bothering you, then it's too loud.
As Zd suggests, if the hiss is not equal in both channels, then that's really a dead giveaway. You likely have a single pair of tubes in the preamp section. If you swap them -- in other words, change which tube is in the signal path for each channel -- and the strength of the hiss changes channels along with the tubes: voila, there's you culprit.
Getting used tubed equipment, it's fair to assume that the tubes are suspect (unless you got some very explicit representations to the contrary, but even then to be honest). Tubes get old, and as they get old they can get troublesome in all manner of devious ways. Sometimes they develop that trademark "tube rush" that it sounds like you're hearing. Usually, a pair will develop this at different rates, making it easier to identify as it will be stronger in one channel than the other. But not necessarily. So, if it's the same in both channels for you, still think tubes should be your first suspect. Sometimes, however, aging tubes wont be so generous as to give you the audible clues. Actually, think it's kind of them when they volunteer. Otherwise, they can just slowly lose steam and go flat. And like slowly boiling a frog, can happen too slow to notice, and then suddenly you're cooked.
All that said, all tubes go bad eventually. The preamp tubes you'll have usually average about 3,000 hours useful lifespan. But like any lightbulb, averages can be meaningless. With used tubes, you have no way of knowing how much life they've got left in them. If any. Thus, I've always found it useful to just buckle down and keep a spare set. Swap them in, see how it sounds. If it cures what ails you, clouds part, and the sun rises on a brighter tomorrow: it was time. If not, put the old ones back in and rest assured that the day will come and, hell, you'll be ready.
