Signal tube versus power tube, sound impact?

I will preface what I am going to type about JJs and EHs by emphatically stating that I do not have the golden ear, I do not own a tube tester, and I make no claims whatsoever to be a tube or audio guru.

My current preamp (SLP 05) came with six EH 6SN7s (two for the left and right balanced inputs and the other four were the left and right gain stages). I had the good fortune of getting numerous pairs of moderately used vintage tubes to roll in the balanced input sockets and I heard significant improvements with some (not all) of them. In the past, what I had been MOSTLY hearing was that the EHs were crap and that JJs were not bad for the price, so I recently took the remaining four EHs out and replaced them with JJs from Tube Depot. I thought that I heard an improvement, but I will admit that, on this, I may have been hearing confirmtion bias.

The generous individual who gave me the deal of a lifetime on those numerous pairs of vintage 6SN7s (in those two balanced input sockets I am currently running a pair of ’52 Sylvania "Bad Boys" that I got from him) also told me that he was done chasing NOS signal tubes (he no longer runs 6SN7 based gear) and from now on he is sticking to JJs for signal tubes, and to quote him as best I can, he told me that for his money, JJs were as good as it gets now. He actually favorably compared JJ 12AX7 sound to Telefunkens.

On the subject of signal tubes...
I like the EH 6SN7, but they absolutely have a very "solid state" sound, totally lacking the tube warmth you’d expect - especially from a 6SN7. They’re a "situational" player for sure. As @immatthewj referenced, this tubes certainly has its detractors! For this very reason. A preamp that ships with a battle squadron’s worth of EH 6SN7 will either have to be voiced VERY specifically for this tube, or it will improve massively from some 6SN7 rolling - which further contributes to the tube's bad rep, no doubt.

EH 12AX7 is voiced to be quite bright, really just too damn bright - I have never found a use for them. I’m flummoxed the Herron VTPH-2A shipped with them. EH power tubes like KT88 and EL34 have a more "fun" and brash voicing; I like them but they’re not for your typical audiophile.

As for the other Russian sub-brands - Gold Lions generally have a refined smooth sound with a more traditional tube voicing, Mullards are always extra warm (with relaxed treble), and Tung-Sols generally have a great balance - good for all genres but especially for rock.

Vintage 6SN7 can be super but they’ll always measure a lot weaker (transconductance) than the modern Russian 6SN7’s (EH and Tung-Sol 6SN7GTB), so I really like the latter in driver slots. The Tung-Sol GTBs are just a little sweeter than the EH, still a far cry from the "sweetness" of a good vintage 6SN7. Vintage Tung-Sol round plate black glass 6SN7GT are super lush and sweet btw. But many vintage 6SN7GT’s also have issues with noise and microphonics, so I’m glad I only have to stick them in power amps - you’ll drive yourself crazy with these tubes in a preamp or (God forbid) phono stage.

. . . what a representative from the sales department at Cary Direct once told me (the Cary people will actually talk to you on the phone if you want to buy something) when I called to inquire about the Amperex Bugle Boy 6DJ8s that Cary Direct was listing:  "Tube rolling will make you neurotic."

That is a direct quote. 

And yes, I did buy a pair of those 6DJ8s for the front of my Cary V12.  I don't recall being overly impressed with any changes to SQ I might have heard, and presently those Bugle Boys are back in the "tube drawer" of a file cabinet I have and I have a pair of Gold Lion 6922s in those sockets.

. . . I also meant to say that although I personally never heard huge sonic differences when I was going back & forth with different HEALTHY output tubes in my first two 6550 based amps (I have never rolled tubes in my present EL34 based amp as long as it was running with HEALTHY tubes), if I had an output tube or tubeS that were getting sick or were getting tired, replacing output tubes was then  like listening to a new amp.

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.