What's causing hiss??

Is the hiss identical for both the left and right channels? Also, do you get the hiss when you switch to an input that has nothing connected to it?

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 won’t 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.

I don't know all of the usual diagnostic steps you may want to take. I do know as a tubophile, that tubes can be the source of a hiss. If you can swap out the tube/s in the preamp section you may find your answer very quickly.
You should also be aware that tube equipment is inherently noisier, not that all tube systems are noisy. What I am trying to say is that if you are very sensitive to background noises some tube systems will never be quiet enough for you.

What Mechans said. Incidentally and apropos of nothing, it's this constant dance of second-guessing and general fussery that sent me - with much wailing and gnashing of teeth - back into the comforting, predictable camp of solid state. Say whatever you like about the sound, tubes are unquestionably higher-maintenance, less predictable, and more like living little bits in their own right, in all of the good and bad connotations that might evoke. Just part of their care and feeding to grow accustomed to, although once you get the right ones in a preamp section, you can usually forget about them for a couple years. Unless, of course, you can't.... ;)

Is the hiss audible at your listening position or just when you put your ear up to the speakers? Assuming the latter, this is not uncommon at all and much depends on the efficiency of your speakers and/or the noise level of your tubes. Get some new tubes tested for noise, i.e. they are graded as 'low noise'. This will solve most of these issues unless you have very efficient speakers that are revealing the noise threshold level of your electronics.