What kinds of tubes I can use?

I suggest checking with the manufacturer on substitute tube types, but, that aside, I also recommend that you listen to your new amp for several months to let it settle in, determine its sonic signature, and exactly what about its sound that you like or dislike. The 6N8P's are equivalent to 6SN7's. In current production I like the Electro Harmonix gold pin versions, but these are readily available and reasonable priced such that you can check for yourself. In NOS versions, there are several web sites that you can google but for me, I have found that Ken Rad's deliver the best bass punch (atma-sphere m60's), RCA (or other) 5692 red base for the best midrange liquidity, and the elusive Tung-Sol round plate VT231, for tonal correctness (way too expensive, and a LOT of used up tubes being sold as new, so beware), and I am told that the Russian military metal base is highly regarded (no personal opinions here). The 12ax7B are available from several chinese manufacturers and are promoted as a deluxe version of the 12ax7 (I can't help you here because I am just not familiar with whether or not the "b" version is a true upgrade. The KT 88's are probably your most expensive tube to replace, so I would not personally change them out until you have to. In any event it MAY (check with your manufacturer) be substitable for EL 34's (better mids) or KT 90's (better bass), however your amp may require biasing the output tubes when changing them, so again, READ your insruction manual and CHECK with the manufacturer! Happy Listening and enjoy your new amp. Sincerely