Mullard EL34 that sounds best

@inna How are those old stock Mullard EL34 xf2’s sound in your VAC? Better than your SED’s?

Mullards XF2s are in my VAC Avatar SE. I'll give them a burn in time of a few dozen hours. But it's clear right away that they are far superior to SEDs in every respect. The most striking difference is how well they present the performing space, how at the same time the sound is more coherent and instrument separation is much better. Drums don't jump out of the speakers and Miles's trumpet doesn't scream in my face. Well, I exaggerated a little, SEDs are not bad. And overall, very sophisticated sound with depth and grace.

In fact, the Mullards that I just got improved further after 100 hours of burn in. Either they were virtually new or were not played for some time. Or both.

"Mullards that I just got improved further after 100 hours of burn in” 

Tube amp need to be biased/balanced after 50hr at least after tube replacement.. was this amp tuned after tube-refresh at all??

I am not sure I understand what you are saying. With this VAC at least you just set the bias for new tubes, then check it a few times later and leave it alone. Recheck every month.