Signal tube versus power tube, sound impact?

I have noticed more difference changing the output tubes.

Power tubes first, small tubes to fine tune. JJ power tubes are OK if you like them. I use their E34L's in a small amp in preference to some other excellent tubes. Power tube selection can make a big difference in the sound so the first thing you need to do is try to define what sound you want to hear. While tubes do interact with your amp to get this sound power tubes do have a intrinsic sonic signature (as do small tubes). FWIW new production tubes are generally good, NOS power tubes are exceptionally hard to find and very expensive. 

Likely only 1 preamp gain tube  in each channel will make a noticeable SQ difference. Could be subtle or quite noticeable.

If your amp accepts different model tubes-EL34 for example, it will be a noticeable move. YMMV, but typically enhanced mids/less bass with EL34.

Look up what other type will work and their sonic character.