I have a couple of pretty old Siemens tubes that sound great, I keep one in the 1st position on my Ceriatone. Somehow I've collected quite a few tubes, nothing fancy, but anyhow at least a couple dozen of them.
I don't find there to be a big difference in preamp tubes used except in the first tube the signal sees. That makes a huge difference, swapping the old Siemens to a Sovtek for instance makes it very much colder and less 'tubey'. But swapping the others to whatever makes no big difference you'd notice if you swap one tube in whatever position - if you use all NOS or vintage tubes or all Sovteks you do notice a clear difference, it gets already too vintage and tubey is everything is NOS, but anyhow I've decided to focus on the first tube as it makes the greatest difference.
I suppose it depends on the type/amp design but at least on mine the preamp n.1. tube is the key to everything. On 6L6 and EL34 based amps I didn't hear a significant difference swapping power tubes either, but on the EL84 Ceria they make much more of a difference. It makes sense, as those tubes are always ran hard & the amp at near full power being lo-wattage, whereas I hardly ever got to crank a JCM800 or 2203 - or them Fender combos.
At the moment I have a Siemens, an old Peavey and Sovtek in my amp. The power tubes are all Sovtek now. I used a bunch of EH's but the sovteks sound better. I also have an octet of mil-spec sovtek EL84M tubes, but those do sound a tad cold. I just keep 'em as emergency backup. They worked fine in my Classic 60/60 poweramp though.
I'm no tube expert, and never have really tested other NOS tubes than my Siemens' ones and some old Marshall etc- nameless tubes - there were a few real old ones in my VT-22 Ampeg, but anyway I've still tested a huge number of tubes, especially in the H&K CreamMachine and ValveJr., enough to be bored of the whole issue already ;-) but my take is, in the end, that you always want a _good_ first tube in your amp and you don't want mil-spec tubes in your amp even though they last forever...more than that seems excessive tinkering to be in relation to changes achieved - much like debating which sounds better as guitar body, plastic, cardboard or mahogany - it's more to do with religious belief than actual changes in sound IMO.
Dee
"When life's a biatch, be a horny dog"
Amps: Marshall JVM 410H w/ Plexi Cap mod, Choke Mod & Negative Feedback Removal mod, 4x12", Behringer GMX110, Amplitube 3/StealthPedal
Half a dozen custom built/bastardized guitars all with EMG's, mostly 85's, Ibanez Artwood acoustic & Yamaha SGR bass, Epiphone Prophecy SG, Vox Wah, Pitchblack tuner plus assorted pedals, rack gear etc. for home studio use.