Re: Any suggestion on tubes??
I've never been a 6L6 amp owner so I won't comment on power tubes.
but for the 12AX7s I'd say go NOS if you can afford it, at least for the 1st preamp tube. Even the $20 "almost NOS" stuff generally sounds clearer to me than the best new production stuff.
But if it's new production stuff, it's up to your tastes.
Go with a JJ short-plates to have a darker sound
go with tung-sol to have a even, clear sound
try electro harmonix if you like a little brighter sound.
the stock sovteks are bound to be the WA, WB, or WC types. I find most of those to sound a little muffled and so yes, I think upgrading would make an audible difference.