I also have a Classic 50 (2x12). I use Sovtek 12AX7 preamp tubes (they
have A and B versions; I don't know which is which but I use the hotter
version). I have used other brands (Groove, Ruby) but keep returning to
the Sovtek's. I'm still using the original power tubes, so I haven't got
any advice for ya there. I did, in the past, have a tube Marshall, and
used whatever came in the package marked "Marshall" from my local music
store; those were good power tubes, but I don't know who made them.
luck
wes
PS: Oh, yeah...you should be able to do it yourself. Take your time. I
recommend you unscrew the power cable retainer from the inside of the
cabinet first. Then hold the amp section with one arm & unscrew it with
the other. Once you get the access plates off, the tubes will come right
out. You might have to give them a little wiggle, but just a tiny bit of
rocking back & forth ought to do it. Be Gentle. When putting the new
tubes in, just make sure you line up the pins with the holes.