As others have said, Tube Pre's tend to go forever, before tubes need replacing. Once you have heard tubes in the Pre or Power amp, there is no going back, as far as I am concerned. It is tempting fate to say this, but I have never had a problem with a Power amp tube going and that has been very infrequently. I have heard they can take out capacitors when they go, never with me. Replacing a tube could'nt be easier to do. To check which tube has gone, simply switch one tube at a time to the other channel, till the problem appears in the other channel and you have your culprit.
I have had a number of Tube Pre amps and I agree with Lowrider, that CJ make some of the best, the 17LS I had for a number of years was one of my favourite components. Unfortunately I need XLR outputs into my ARC Ref 75SE and CJ do'nt do balanced outputs, so I bought a Modwright 36.5 as an ex dem model and it is excellent, as good as the Arc Ref 3, in my opinion and a lot cheaper.