Signal tube versus power tube, sound impact?

I have noticed more difference changing the output tubes.

@ellajeanelle This has been my experience too. Especially recently, swapping from KT88 to KT120 (in amps that support this) effects a huge difference. Swapping between different brands of KT88 also makes a large difference, though not quite as much.

That said, yes small signal tubes can make a significant difference as well. Especially whatever tubes are tasked with driving multiple KT tubes.

Many folks claim small signal swaps are more cost effective since NOS / vintage types are still reasonably available, and these tubes can last several times longer than their power counterparts. And certainly, amps that use an octet (of more) or KT88 (or whatever) really rack up the tube bill! I just swapped an octet of Tung-Sol KT120 in place of Gold Lion KT88 for a VAC statement 450S. Made a HUGE difference. I also swapped its sextet of 6SN7’s (mix of NOS and new). That made a smaller (but still large) difference - and yes I tried many various combinations! In other amps I have preferred Gold Lion KT88 over KT120. And EH KT88 also sound different from GL KT88, despite being made in the same factory - just a matter or personal preference (GL are voiced more "audiophile").

The KT88 and KT120 are close enough to be reasonably compatible (bias, loading etc), but different enough to sound quite different. It’s a fun swap :)
Just clear it with your amp maker first. Anything running a KT120 near its limits would fry a KT88, and KT120 draw a bit more heater current from the power transformer.

As for JJ - I had a run of bad JJ encounters in the past (~15 years ago) and never gave them another chance since then. My rule for the longest time has been to bin JJs immediately, whenever a component arrives with them inside. 

Although I do hear that the JJ tubes are more durable and reliable which is why I guess they use them as stock tubes.

Says who, JJ? 😂

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.