Can you do anything to make power tubes last longer ?

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.

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...

Right. I wouldn't like to end up with too many tubes.

Yeah, people chase vintage tubes usually for a good reason.

XF3/XF4 as a spare set..that's not a bad idea at all.