A few basic tube questions

I have a Ayon tube amp, and regards KT-88 power tubes, here are the observations I have made... Genelec Gold Lion tubes are fairly unreliable (blow readily) while the Chinese tubes, those from Grant Fidelity (a fine company to deal with), the Psvane tubes (a bit more expensive than the Russian Genelecs) seem to be reliable. As for sound... the two tubes are, essentially, identical in terms of image size, image specificity, air, and fullness (palpable presence). The Russian tubes (Genelecs) have slightly tighter bass, are slightly more dynamic, and a tad more lively... but are more grainy sounding (harsher with digital recordings), not as smooth or relaxed, or natural sounding as the Psvanes. The Psvanes are more organic sounding, less fatiguing, and generally better. But like Lostbears says, changing out the tubes will not "redifine" the sound... though the differences in brands (at least Psvane vs. Genelec) does make a difference that is jnoticible. Over time, I find the Psvanes to be superior to the Genelecs... albeit, not by staggering amount... it is somewhat subtle, but meaningful and more than warrants the price difference. If you go the KT88 route, go Chinese rather than Russian.

I would buy NOS tubes (preamp or power) from reputable sources like the ones you listed. I've also had a good experience with buying from Jim McShane as well as Brent Jesse. For new reissue tubes I've had good experience with Doug's Tubes. These sellers do a better job of testing and matching etc. Buying used can be a bit of a slippery slope. If you are lucky you save some money but if you aren't you may end up spending more money than if you just went to tubemonger etc at first. I've been burned buying used tubes from non reputable sources. Most of the time they were noisy or microphonic.

Start with getting new powercords. I put 5 Nordost Blue Heavens in my system and they made more of a difference than tube rolling my amp and preamp. Just try to get matching tubes then keep a back up(full) set plus one or two more tubes handy in case one or two fail. Best of luck.

Lots of good advice so far!You really do need to let the system settle in for a while.Then after a lot of Googling about specific tubes and their sounds,change one type in one component at a time.Run the system 3 to 5 hrs. on then let it rest for 3/5 hrs.when you get to 30 hrs. any grain or glare should be gone.If you like the change,move on to the next tube type.By mixing brands you can get the perfect combo for your system and tastes.It takes much patience but will be worth it.Brent Jesse is great,the Tube Store,Grant Fidelity.
Best of luck.