My experience is like @ghasley. Small tubes in pre stages made way more noticeable change than output. Using NOS recommendations from both maker (Raven) and Brent Jesse I chose military and or scientific grade pre tubes. Stage expanded and bass tighter at every position tried. I ordered unit with one step up in 2 of the 3 positions too. Most run cheaper than output tubes. I then tried better/desirable/ recommended output tubes havibg been so happy but its much more subtle a change. YMMV. I set a budget and said I'd stop if if I didn't hear any improvement with each position. But it did improve. Done now. 2 years "sober". Not neurotic. Besides, I now have perfectly decent spares.
Signal tube versus power tube, sound impact?
My mono block has eight KT 88 tubes(power tubes) and six smaller signal tubes. I'm using the stock tubes that came with each Mono. I think they are JJ tubes.
Rather than replace them all would it be best to change out the signal or the power tubes? Not sure how good the JJ tubes are but I hear they may not be the greatest.