Mullard EL34 that sounds best

I would recommend JJ power tubes https://www.jj-electronic.com/en/power-tubes

pro: very stable over long run time

Luxman Japan uses those for all noble tube amp designs 

The Avatar is a great amp. The new stock Mullards are very nice sounding tubes IMO compared to most. Do a google search and try to place the reviews of different tubes to your preferences. 
 

It appears that the older Mullards you go with the better, generally speaking.

The price from reliable sources, if they have them, and even from unreliable ones, is very close to prohibitive. $450 for XF1 each new or $300 for used, that's what I see. Some say, though, that NOS EL34 Mullards might last 5000 hours, which is great if true.

(any) tubes will last longer in gentle usage conditions, lower dissipation power / current / voltage increases ~ 20% spec degradation time point. to figure out how long tubes will last need to check amp design and tubes aging related spec. 

I just tried some New Sensor Mullards in a headphone amp. Maybe it was the circuit but I couldn’t wait to take them out. Nothing like I remember when I ran them in some mono-blocks several years ago. I’d stick with SED’s if you can still find them. That is unless you’re ready to mortgage your first born.

I remember reading here where someone liked the Sofia Electric EL34 more than original Mullards.  Can’t remember the thread or who said it though 🤔

My Cary amp uses a dozen EL34s and NOS or used vintage is way out of my price range.  The first time I retubed it I bought the Mullard re-issues from Viva, and they didn't seem to hold up very long, but that may have been partly my fault.  I replaced those with the reissued Tung Sols from Jim McShane, and I personally liked everything about that batch better.  Sorry, as I know this was not an answer to your question.

Oh it's fine, we are just talking tubes.

Tube Depot has a stock of SEDs EL34, I bought my quad from them. One was defective, only held for 700 hours.

Then there is a seller from the UK on ebay who sells, as he says, original Svetlana winged 'C' s EL34. They look a little different. I'll try one of those later too.

I have always read that caution should be used when buying vintage used or theoretically NOS from Ebay.  My own personal experience was buying some RCA black glass VT231s from some guy in France, and two of them quit working not too terribly long after I took delivery, and I took the rest of them out and put them in storage after that. 

Yeah, but he has an excellent and substantial feedback, and each tube is under $100, so I'll give it a chance. One tube to begin with.

Just can't spend that much on new Mullards from Tube Depot, though they have some other Mullards for $300 each new.

Besides, they only have a few tubes in stock, so it is not a long term solution at all.

Have you thought about trying Rays tubes?  I have not but if you believe the reviewers they sound very good.  Around $850 per quad for their reserve quality and $300 for the select quality and they’re warranted for 12 mo.

Never heard of them. "Sound very good" is not good enough for me. Either I will stay with SED Winged, that are alright, or will want vintage Mullards.