How do you know when preamp tubes go bad?

Over the last few days the sound quality of my system has diminished considerably. I'm using a tube preamp driving a SS amp.I've seemed to lost a lot of my my soundstaging.Any input would be greatly appreciated.
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.
I pretty much agree with Mezmo's observations, and would just add that when the 6 tubes in my pre-amp (SF Line 2SE) get to be 12-14 months old, music quality starts getting a bit hard, bright, strident, and progressively more fatiguing. I keep the pre-amp in standby when not in use, and this uses up some tube life, and then I listen to music 3-5 hours per day.

With new tubes, the hardness and slightly fatiguing music character goes away. I've used this pre-amp over four years now and have changed tubes several times (not counting experimanting). Perhaps this (short)1 year tube life has to do with the stock Sovtek 6922 tubes I had been using. BTW, I recently switched to a set of NOS Sovteks and realized a significant improvement in music quality-- hope they last longer than the stock tubes. Cheers. Craig
Usually the sound gets thin, dull, and lifeless, when tubes are nearing the end of their useful life. You lose dynamics, the bass get soft, and loose, and the highs seem rolled off. Most times, it is so gradual, that you don't notice, till you install, new tubes.
Sovteks usually last for 1-3 years, depending on how much you play them. NOS types, tend to last 5-10 years, under the same circumstances. I think most NOS tubes, are sonically superior, by a long shot, to Sovtek tubes.
I'm no expert, but I thought they started to "ring" at the start of their demise. The ringing would go away by tapping the chassis top but randomly return weeks later. I'm currently running an Audible Illusions 3A with white Amperex 6922 (NOS).
I read somewhere that Victor at BAT said an old tube can be reactivated by feeding it a certain current at certain voltage. Does anyone know how to do that?
you know preamp tubes have gone bad when they call you at 2am asking to be bailed out of driving a solid state amp...

some humor never hurts...
this is not intended to stir tubes/ss each their own