Ordered a pair of matched JJ EL34 tubes and swopped out the Marshall stock EL34 ( will keep as spares )
Replaced V1 and V2 with JJ ECC83S
Pleasing results as it sounds warmer, fuller and less harsh at high gain which the better JJ tubes seem to be helping
A nice easy upgrade
I did a complete revalve on a Fender Hot Rod once. There was a small difference (slightly warmer/less harsh), but it was changing out the Eminence speaker for a Jensen that really made a big difference for that amp.
For me it has to be Ritchie Blackmore and his stratocasters. As far as I can tell he has at least two - one which is a pale natural wood finish, and another which is painted an ivory colour.