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 

(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. 

atmasphere + 1

JJ tubes are built at former Tesla factory, which supplied tubes military/aviation etc.. in 80s Tesla EL34 were the best I’ve experienced, reliability performance etc. not surprised to see JJ tubes in Luxman designs, 3yr warranty is not easy to deliver!

Typ I order tubes and ask seller to make sure serial numbers are close, ideally from the same lot.. I haven’t seen drive mismatch issues even after 1000hr run!

Beware, in many cases like-new/new NOS "Mullard/Telefunken/etc" tubes are counterfeit-miss-labeled Chinese ones..

"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??

correct, tubes initially age at faster rate, thus first 500hr tuneup is needed 50hr-100hr-200hr-500hr.

what I like about JJ tubes is slower aging pace, less tuneups needed, and I suspect therefore Luxman selected them for their amps.