New to tubes

Wow Marco, I didn't mean to stress you out: three postings :). Not sure about the KT88's. Could it be EC 88's? Does the type of tube matter for the lifespan? Maybe a tubestester is a good idea, although I might be a little paranoid, since the tubes installed are about half a year old, and I even have the original ones (which were replaced for better sounding ones, according to the seller). Is a tubetester easy to get hold of? And how much would such a beast cost?

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.

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.

Marco, who said anything about prostitutes? I think my girlfriend wouldn't be to happy.... :)
And Musicslug, actually there are only two tubes in the pre, but I still can switch them, so your idea is valid. Although, with two tubes probably the same age, it would be easier just to replace them both I suppose. And TJ, since tubes ar more expensive then oil, I don't think I'll replace them that much, besides, I just got 'em, so I'm still rediscovering my music at the moment..
Just one other remark: first thing I'm going to change is the cheap, plastic offboard psu (12V/500 mA) for something homebuild and beefier: 12V/5A. I guess that will improve the pre a good deal.