Signal tube versus power tube, sound impact?

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.

Going on about 30 years ago I spoke with Andy of Vintage Tubes (in Michigan) with basically this same question. I was interested in retubing (with NOS) a pair of ARC VTM 120s (6922s and 6550s) and a Cary SLP90 (12au7s).  Andy's advice (which I followed) was to start "closest to you" which meant the preamp and then the signal tubes in the amp.  The improvement in SQ was significant.

A few months later down the road I talked to Andy again about output tubes, and he wound up selling me a pair of quads of NOS Tungsols (not the Russian reissues that are out there now) to replace the Svetlanta 6550s I had in them.  For as much money as I paid for those NOS Tungsol 6550s I was expecting a huge sonic dividend and I was disappointed.  I really didn't hear any improvement.  Maybe because I was expecting too much?

Anyway, going by that experience and Andy's advice, I would start replacing tubes from the front and work backwards.  And I have heard good things about JJs for the price.

We have one answer that says power tube> signal tubes and another that says signal tubes>output tubes.  Hmmm.

@hilde45  , I am only relating to OP what I was told by Andy of Vintage Tubes many moons ago, and what my own experience was with replacing Svetlanta 6550s with NOS Tunsol 6550s.  I do not claim that either Andy's advice or my own experience is gospel that will apply to everyone. 

We have one answer that says power tube> signal tubes and another that says signal tubes>output tubes.  Hmmm.

. . . and actually, it looks as if it was two answers for outputs>signals versus one answer for signals>outputs. . . .