I've owned a Classic 30 for the last 2 years (just bought my first G&L though
) and a Classic 50 for 4 years before that. I highly recommend the very active
Peavey forum. There are a couple of know-it-alls over there that might hassle you for posting w/o searching but otherwise a good group. The group-think there seems to prescribe the following:
V1 Tung Sol
V2 ECC803S
V3 Sovtek LPS
It really depends what you are looking for but I'd support the V1 & V3 recommendations and say V2 being what colors the distortion on the lead channel is really up to you, you can lower the gain here or do whatever.
I've used JJs, EIs, Sovteks and Electro-Harmonix for power tubes. I'd say stick with the JJs or EIs. The consensus is these are the sweetest and chimiest of the bunch. That said the EHs I held up for 2+ years and stayed pretty consistent under the punishment I put them through, using an attenuator and diming my amp.
As for the speaker there are two camps, break in your Blue Marvel or replace it with Weber, Celestion or Eminence (don't ask me which models, you'll have to search the forum). Most agree that the stock Blue Marvel lacks as is but some feel all it needs is 40 hours of break-in either by playing through it, or piping white noise or moderate rock recordings through it. I personally have the head version of the C30 and play it through a Valve Jr cab which has an Eminence Lucky Lady. I didn't personally warm to the Blue Marvels in my Classic 50.
If you truly want to mod it
Blue Guitar has a few. My biggest reason for possibly doing one of these mods on mine someday is the distortion on the lead channel can be a little harsh for me. In the meantime I simply roll off my tone knobs just a bit and/or set my treble below 7 or try to EQ it out somewhere else in the chain.
Seriously, I'm no expert. Check out the Whammy Bar and Grille. In the meantime I'm falling in love with my S-500 Tribute. That thing through my Classic 30 is like honey.