Hopefully you'll get some more sophisticated advice than mine, but my experience has led me to believe that the only real way to figure out when and where you've got tube issues (save some fancy gear you neither want nor have) is through trial and error. On the rare occasions that I've had a tube go sour on me, it hasn't gone quietly. Which is to say that one in particular got noisier and noisier until I decided to yank it. In that case, the noise will be in one channel, so you know where to start. Switch the tubes between the channels in pairs, one pair at a time, until the offending bits move, and there's your bad apple.
What you describe MAY have to do with another issue -- that, towards the end of their lifetime, tubes just start to roll off and loose their juice. Tubes in a preamp should be good for at least three years (at the very least) before anything like this starts to happen. Even then, it would be fairly unlikely that all of the tubes would go at the same rate and at the same time, ideally giving you some sort of audible artifact that you can pull the above channel trick with to see where the bad apple(s) are.
The easiest, and perhaps most practical, approach when you feel that the whole tube compliment may be holding you back is to retube the whole thing. Get some NOS glass, try a little tube rolling, and see what makes a difference (likely not all that much of an expense when you're talking a preamp). If things open up with new tubes, clouds part, and the sun dawns on a better tomorrow -- then you know. If there are subtle differences and balances of better or worse with different tubes, then you've come to the dance and can choose to voice your own system to suit your own taste. If there is no difference at all (which is fairly unlikely) then, hell, you've got an extra set of tubes (which you'll probably need eventually anyway) and it's time to bark up another tree.
Finally, from a more removed perspective, a relatively sudden, uniform (in both channels) sound degredation doesn't really sound lilke a tube issue at all. That said, I don't have a better idea, so who knows? (Not me, at least). Best of luck and remember to have fun.
What you describe MAY have to do with another issue -- that, towards the end of their lifetime, tubes just start to roll off and loose their juice. Tubes in a preamp should be good for at least three years (at the very least) before anything like this starts to happen. Even then, it would be fairly unlikely that all of the tubes would go at the same rate and at the same time, ideally giving you some sort of audible artifact that you can pull the above channel trick with to see where the bad apple(s) are.
The easiest, and perhaps most practical, approach when you feel that the whole tube compliment may be holding you back is to retube the whole thing. Get some NOS glass, try a little tube rolling, and see what makes a difference (likely not all that much of an expense when you're talking a preamp). If things open up with new tubes, clouds part, and the sun dawns on a better tomorrow -- then you know. If there are subtle differences and balances of better or worse with different tubes, then you've come to the dance and can choose to voice your own system to suit your own taste. If there is no difference at all (which is fairly unlikely) then, hell, you've got an extra set of tubes (which you'll probably need eventually anyway) and it's time to bark up another tree.
Finally, from a more removed perspective, a relatively sudden, uniform (in both channels) sound degredation doesn't really sound lilke a tube issue at all. That said, I don't have a better idea, so who knows? (Not me, at least). Best of luck and remember to have fun.