First tube roll: Which to change?

I'd recommend the preamp tubes first (12AX7, 6922). The 12AU7's may also be used in the preamp section, though they could be used as drivers too. I've found that changing the preamp tubes of a tube integrated amp affects the sound more than changing driver or power tubes.

Cheers,
John

The best bang for your buck is the 12AX7 as this is the first tube that the signal sees

I am guessing you have an Onix sp3. I found also that the 12ax7 position has the most effect on you tonal balance and flavor of the sound. The sp 3 has enough gain to sub a 5751 in this position and some of these really sound nice. I really like the GE black plates in this tube. In the 12ax7, I like the Groove Tube 12ax7m, but these are really hard to get. These bring back some of the classic tube sound to the amp. Also, get yourself a quad of Valve Art 350B. These babies make this a different amp entirely.

IMHO, I would really suggest that you just listen to the amp for at least a few months to let it break in and to get some idea of how it sounds, from bottom to top, and how it images. Also, how quiet it is. Find out how it sounds at low volumes and also at higher volumes. How much detail does it flesh out at low volumes, for instance? Then, AFTER you know what it sounds like, can you then make any judgements about tube swapping. Just my two cents worth, Happy Listening, and congratulations on your new amp. Sincerely,