Signal tube versus power tube, sound impact?

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. . . .

Start with the small tubes/preamp tubes. If you go with power tubes first, you will correctly wonder if you are hearing them for what they are...and the answer is no. Marginal signal tubes fed to exceptional power tubes stands a better chance of remaining marginal. An exceptional signal tube sent to marginal power tubes stands a better chance of improving things. If you feed your amplifier the best signal possible, then you will be hearing them at their optimal. My 2 cents and most likely...you will change both, otherwise, it will feel like a Larry David skit.