Tubes most likely to improve SQ?

Agree with the preamp tubes first but the thread should read "most likely to affect SQ"....whether it's better is up to your ears.

RAM EL34's are rich and smooth sounding.  Highly recommended.

Tubes will respond and sound different in different circuits, so YMMV. The best 12AU7 I’ve tried in my amps are the RCA clear tops. I’ve also been really happy substituting a 12BH7 for a 12AU7... love those.

Different el34s can sound quite different too. In my amp, the Electro-harmonix EL34s have wonderful clear mids and treble. The Nesstone EL34s have tighter bass. (since I’m bi-amping I can use both where I think they sound the best)

All of the tube slots should affect the sound, and I imagine it would be mostly in a positive way for vintage tube swaps. Yes start with 12ax7 - it’s the most likely to make the most impact, and there are gobs of them out there. Then 12au7 - like knotscott I always got big gains swapping to 12bh7 away from 12au7. When it comes down to it I don’t really like the 12au7 tube type that much. HOWEVER the 12bh7 draws twice the heater current, and your amp’s filament supply needs to have enough overhead to supply this -- it’s really going to be hard to verify this on a Chinese built amp, and I definitely wouldn’t assume it’s OK. But there are other 12au7 subs you can look into that don’t suck down much more filament juice than a 12au7. I also find power tubes make a big difference too, but the vintage market on these is extremely small and expensive. Best here to stick with Russian re-issues, and try a different compatible "type" that your amp can handle (e.g. KT88 to KT90 or KT120 is a common sub).

And don’t go too crazy spending too much on vintage NOS American / Euro tubes for a Chinese tube amp! Classic "used, tests strong" examples from the big makers (RCA, Telefunken, Amperex, Mullard) should do nicely here. The "big", well-known NOS tube dealers will charge a boatload for matched pairs -- you'll end up spending a good portion of your amp's actual cost before you know it. You could get unlucky with a set of duds from the secondary dealers/markets, and still come out ahead.

Bought Hammond vintage 12AX7A's and 12AT7's from Brent Jesse for my Jolida 302a, noticeable improvement, but biggest change was Sophia Electric EL-34's, much lower noise floor, excellent bass and voluminous soundstage, well worth the price.  Two have failed since purchased 1 1/2 yrs ago, but I often run the unit 6-8 hours per day.  I will replace the small signal tubes with Sophia Electric as well in the future. Highly recommended!