Amplifier Power tubes - important to sound?

I’ve always found power tubes more important to shaping the sound in ways that are meaningful to me. Small tubes can also be important to be sure, but a lot of times it’s an initial "wow!" impression, then over time I either tire of that variant or just want to hear a different flavor. I listen loud (more strain on power tubes), so perhaps that explains my findings.

With power tubes, my initial impressions tend to stick. The change from a KT88 to KT120 is especially significant in amps where I’ve tried this swap. Anyways, what’s your tube amp that has 16 freaking KT88 slots to replace with a non-existent Siemens 6550? (I know op won’t answer) To be fair, if the Siemens 6550 did exist, I’d prefer it over JJ.

I actually have a pair of VAC Master 300 that use 16 KT88 (plus 8 6SN7), and even with auto bias it’s a kind of a PITA to swap. The KT120 sound great here, too.

@mulveling 

Yes I am looking for siemens 6550. It does look elusive.  And will have to revisit the source of recommendation whom is very good at what he does. I like to biamp so I'm using MC 901 monos from McIntosh.  I like them a lot and they blend tube and solid state very nicely.

I do like VAC quite a bit and still think about it. 

I did a lot of rolling on this class tube, kt88, kt66, 6L6, 7581a, 807A, russian tubes I don't remember the number of,  and they all sounded different.  It seems invariably I liked the new sound (human nature) at first but after going thorugh perhaps a dozen sets of tubes, I ended up back at the tube that the maker (Decware, Steve Deckert) had designed the amp around.  Makes sense really.

Jerry

@emergingsoul I think that you are Hi-Fi hero to go for right-away upgrade before even turning the amp on or defining goals for replacing 16 working tubes.

Are you sure you don't mean Sylvania 6550?  Can't say I've ever heard of a Siemens 6550, and I've tried pretty much every NOS KT88/6550 variant out there over time.  And some google searches yield zero hits. 

Yes, going NOS 6550s will likely make a notable difference in sound (knowing the amp in question will help folks answer), but the question is whether it is worth the cost.  If you want to go NOS, the GEC / Genalex have the best reputation (they were fantastic when I used them in an Air Tight ATM-2 - by far my favorite variant), but in the volumes  you are looking at are basically unobtanium as NOS (if you need, say, 4 matched quads, you are looking at easily over $10k, and likely over $15k if you want something that is truly NOS and not just "tests as").  So substantial difference, but probably not worth the price.

Of the 6550s, you may be able to scrounge up 16 NOS Tung Sols in the grey plate variation (which are also nice sounding tubes), but you'd likely be looking at >$6k just for the tubes.  Again, 16 Tung Sol black plates (the best sounding variation) are also likely unobtanium - TubeDepot is selling singles for $1k per tube).

In the more "reasonable" cost bracket, you are looking at GE or Sylvania 6550s.  That will probably "only" run you ~$3-4k.  But the question is whether that cost is worth it over something like the current Russian Gold Lion KT88s in your amp (which tube is better than the JJs, but of more reasonable cost than the NOS options). Or you could look at something like the KR KT88s.  The EATs are nice but no longer made and also very hard to find.  Even the Yugo type ii Ei KT-90s are getting ridiculously expensive - I have seen quads listed at $1700+.

In short, whether NOS power tubes -when you are talking 16 of them for your amps, are at all worth the difference is highly system and wallet dependent - which you haven't told us anything about.  In the abstract, I think even moving from the JJs to the Gold Lions would be a nice little upgrade, and depending on how much difference you hear, you can then determine whether it makes sense to spend the serious coin on NOS.