Mullard EL34 that sounds best

(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.