Can you do anything to make power tubes last longer ?

Sure, that’s a fine strategy - stick to seeking what you really want. But I’ll say that even XF3 and XF4 Mullards were much much better than SEDs, to me. I didn’t really like SEDs; they came off as a 1-trick pony for lots of warmth, lacking the articulation and 3D holography of British Mullards. That seems to be a common pattern with modern tubes, or Solid State components that try too hard to emulate the vintage "warmth".

You could get some nice XF3 / 4 to use in the meantime, and then they become great spares when you get those XF2’s :) This is how I ended up with tons of tubes lol...

Really, don’t overthink it. The lifespan will be determined primarily by the amp’s operational parameters (bias, voltage), and quality of the tube. Enjoy your tube amp. Vintage tubes tend to be very good, and long lived. But these days, most of what’s left there are used and will have unknown hours (many???) on them already. Russian power tubes have been very good for me, but they do fail occasionally, and yes "spectacular" failures (catastrophic short) can happen and take out a resistor. Replace those tubes every 2000 hours, at most.

Some of the amps with individual tube bias pots can be run colder or hotter, to get either (maybe) more tube life or more "class A" respectively.

Don’t throw a blanket over a tube amp, but don’t bother with useless tube coolers either. You’d only need a fan if you’re sticking it in a cabinet.

Don’t leave it on for a few hours just to save an on/off cycle. Just turn it on when you’re ready to listen. Unlike some SS amps, it should be at its best in 10 minutes. Any hifi amp you have today should be starting the tube heaters up well in advance of the high voltage, so it’s a safe start for the tubes. You would be hard pressed to find a tube amp anywhere that doesn’t have SOME mechanism for soft start.

I am not going to use any new reissues, only interested in vintage tubes. I see no reason to just waist money on inferior sounding and less reliable tubes. But if you need 20 power tubes, yeah, you just might be forced into that. I only need a quad.

For bigger KT tubes, it’s extremely hard to find good vintage ones anymore. There are still some late make GE 6550s out there. EL34 (like your amp uses) and 6L6GC is much more feasible for sure. The British Mullards are warm (like your SED / Winged C, but better); I preferred Amperex / Holland EL34 for being more a bit more neutral while still sweet sounding. The East German RFT / Siemens are supposed to be quite good too. Electro Harmonix EL34 is a copy of this tube, at least superficially - but the old RFT build quality was much better (straight glass bottles, even!).

I’m quite fond of many of the Russian power tubes so it feels wrong to call the "inferior". But yes, the old power tubes were generally better. The old EL34 were beautifully built.

Also remember - part of VAC’s "secret sauce" is Kevin’s voicing expertise / craft, and it’s usually advisable to stick with tubes that sound at least similar to what it shipped with. What does VAC recommend for an Avatar these days? They were shipping Shuguang KT88 power tubes in 2000s (some with Penta Labs labeling), but Shuguang is long gone. I have a VAC 450S that originally shipped with those Shuguang KT88. They wore out before I got they got. Russian Gold Lion KT88 didn’t sound good in it, but fortunately Tung-Sol KT120 do :) Now that I think of it, might be worth tracking down some Shuguang KT88, sometime.

I also agree that there is no need to use white gloves. I’ll admit I’m an outdoor guy who doesn’t use lotion so my skin tends to be dry. but most tubes run cool enough that a little skin oil it isn’t a problem.

Agree on the gloves point. Actually, this is something that the engineer in me screams "you're solving the WRONG problem!" about. There is NO risk to tubes from skin oils. Audio tubes are nothing like those awful halogen bulbs from the 1990s. The biggest risk to a tube is DROPPING it. And guess what - those cheesy, cheap white cotton gloves are generally very slick compared to a natural grip. So while purporting to protect tubes from (harmless) oils, they actually introduce a REAL risk to your tube handling. Ugh. These gloves should all be burned. 

If you have auto bias, you’re out of luck. I’ll never buy an amp with autobias.

While I do love the ability to manually adjust each tube bias point, the VAC iQ auto-bias system has been very, very good in my use and sounds excellent. 

didn’t like Brimar 12AX7 in the phono and thought that 12AU7 would not be too different, and Sylvania is thought to be bright by some.

Sure - the sonic traits of one make/vintage of 12AX7 generally carry over to the corresponding 12AU7. Sylvania tubes seemed to change a lot from the 60s onwards. Before 1960s: nice clean airy sound, great dynamics, with a beautiful midrange that was romatic and organic while not so thick & warm as Mullard. Afterwards - a more bright and solid-state like sound. Ending with Philips ECG tubes of the 1980s (ugh). The later tubes are certainly serviceable, and even good in some roles, but older Sylvanias with black plates are generally much more desirable and sound good almost anywhere. You might think of older Sylvanias as (sort of) like USA’s Siemens.

XF2's are excellent. I had a quad but they were well worn when I acquired them. They were perhaps a bit more articulate than XF3 and XF4, but don't sleep on the latter - it's far easier to score a really nice quad of those, with a lot of life left. 

Found them ! Matched quad of Mullards EL34 XF2 is coming this way. It’s a little risky because they are not new and the exact number of hours on them is unknown, though supposed to be low enough. Test as strong NOS, as the seller with excellent feedback says. It’s always a gamble with tubes to a degree, I feel fine about it.

Nice! Enjoy - great tubes. Don’t worry about the exact hours on ’em. A meaningless metric, now. "They don’t make ’em like they used too" applies, etc and yada yada...

For Amperex / Hollands I’ve only used the xf4 and xf5 models, both with the brown/red-base. They both sound equally great - I’ve even mixed them in quads (matched pairs), with no problem. The older metal base Philips / Holland version is supposed to be extra special - I’ve almost pulled the trigger on these in the past, but never did.

Perhaps, I should also try Sylvania 12AU7 black plates from 50s as well. What difference would they make compared to RCA black plates from 50s, I wonder ?

Why not? I expect 1950s black plate Sylvanias should be very good. I generally prefer these over any RCAs, though I haven’t tried the 12AU7’s. You can do 5814, 6189 and 5963 subs too. The 5963 tend to be microphonic, so those should be relegated to amp driver slots only (NOT the preamp or phono slots).

RCA black-plate 5963 was the favorite 12AU7 sub for early 2000s-era VAC power amps.

Some claim that solid state requires less maintenance because you never have to replace tubes, but changing a tube is so simple that finding a bad transistor is far more complicated.

This is true. Across probably hundreds of components I've used in my time in this hobby, once you eliminate simple "replace the bad tube" issues, I've had decisively more failures with Solid-State - and they're far more frustrating and difficult. Especially when the transistors you need are long OOP and counterfeited like crazy.