New to tubes

I just got a Musical Fidelity X-pre preamp, which contains two tubes (KT-88's I think). How do tubes age? Will I hear a deteriation in sound? Or will this go so slow my ears will adapt to it, so I won't notice it? Or will they just quit working when there too old?
Basically: how do I know when it's time to change them?
Thanx in advance.
Thanx for the reply. May I bother you with another question, not about tubes this time? My poweramp is a real class A-device, and runs extremely hot. To the best of my knowledge, this should shorten the lifespan of the transistors (Sanken). I have the opportunity to buy a couple of them, but on the other hand, the amp is working (extremely) fine, so maybe I'm just wasting money, and I know zillions of ways to waste money in ways that will provide me with instant pleasures... Any input (not on instant pleasures though) would be appreciated.
Tube Pre and Class A ss power amp-I really like that idea .
Keep it simple change the tubes every 2 years,cost on 6922/EC88 minimal.I agree 5000 to 10000 hours life span.Try some different brands,fun to do.NOS is good but beware paying to much.I am using Siemens CCa's in my Pathos Logos,works nicely!
I'll second the Tube pre and SS amp! (AR LS2B Mk2 and 3 Threshold S-300's!) As far as tubes are concerned, have some fun. Each tube will have a different sound and it is kind of fun fooling around with them. I agree with Marco, find someone with a tube tester or get one for yourself.

You will find the prices will range from low to outrageous. Decide on a budget and start buying some. You can usually recoup most of your cash in the resale market, if you purchase wisely. This will allow you to try many different tubes.

Hop over to and talk with some of the tube guys over there, they can be a real help choosing. Welcome to tubes, it is quite and experience!
Ok guys, thanx for your help. There's nothing deeply philosophical about my set-up though. I changed speakers, but they couldn't get along with my ss-amp, so first I bought this huge class A power-amp, using my old integrated as pre. When funds allowed (or actually, they didn't....) I bought a secondhand pre-amp, which contained tubes, and combined great, so I decided to keep it.
I guess, since there is plenty of life left in the tubes, I first upgrade the interconnect between pre- and power amp, and then I might have some fun with tubes.
Sorry I couldn't chime in again last night Satch. I went to bed with a hangover...not because of your questions though. That may explain all the additional copies of my post! Yes, I'd concur; tube pre with class A SS can be a nice combo when done right. Hang onto your tubes, save your pleasure money for the prostitutes and enjoy the music!