Well, Carvin was selling off some Celestion G12P-80 speakers for $29.99 about 2 years ago, so I bought one, and installed it in my Classic 30. Now that amp sounds like it's supposed to sound. The improvement was even more than I expected. I sold the Blue Marvel speaker on Ebay for almost as much as the Celestion cost me.

The only other mod I did was to install one of those gadgets to keep the power tubes (four x EL84) non-microphonic - it consists of a piece of 1/2" thin-wall conduit with one small hole with a high-temp grommet for the tip of each tube. The tubing is held in place by springs at each end. Bought this gadget on Ebay for $20. It eliminated the microphonic problem instantly.
I have two other amps:
1) A Marshall VS65R, which is a hybrid since it has one 12AX7A tube in the preamp section. Very nice amp, but it has developed a noise problem lately - tendency to make snap crackle pop noises, usually during the Pastor's sermon, of course. So it went home, and the Classic 30 took up residence at the church.
2) A Crate 15-watt XT15R that I bought when the new Guitar Center opened a couple of years ago. All solid state, of course, but an impressive little beast for the $50 I paid for it. It has some digital effects built in, and the reverb ain't bad at all.