Can you do anything to make power tubes last longer ?

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.

It would certainly be interesting to try vintage Amperex EL34 but I am currently on the hunt for Mullards XF2 and getting close to them. XF1 would be much more expensive and even more difficult to find. Then there are first generation Mullard EL34 with metal base that some people consider the best. Well, probably impossible to find a matched quad or at least pairs in excellent condition, so I'll just forget about it. Unless I get very lucky.

Yeah, probably "inferior" was a wrong word not always accurately reflecting the difference.

Of course, SED Svetlana Winged are a copy of Mullard, sound signature should be similar.

When I bought the Avatar I talked to Kevin about tubes. He recommended selected Chinese 12AX7 for the preamp and phono that he had, Brimar or Sylvanis 12AU7 vintage for driver and, yes, Mullard or Gold Lion vintage for power tubes. But I didn't want to go with any Chinese, I 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. So, I disregarded his advice and went with 12AX7 Mullard long plates and 12AU7 RCA blackplates, and I am quite happy with the result. But for power tubes, yeah, Mullards are going to be installed soon enough.

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. 

I see. Before 60s Sylvania were good, I’ll keep it in mind, thank you. Kevin didn’t specify, by the way, he just said Sylvania and Brimar. Brimar lacks Mullard’s sophistication and has a recessed midrange, both are no-no to me.

No, I’ll get XF2, believe you me. I made a mistake after I bought the amp by getting SEDs instead of seeking out Mullards. I’ll correct this mistake. I should’ve listened to Kevin about that and had more patience.