Interesting topic....Lately I've been looking at the same thing.
I haven't made any changes yet, but I've spent a fair amount of time researching the Celestion line of speakers. It seems the V30s are pretty much the standard for rock (Slash is a famous endorsee, among others). If money's no object, they've just released the Alnico Gold speaker, which is a 50w version of their famous Alnico Blue speaker. Apparently it's the tits, but it will be very expensive.
Personally I'm tempted by the Century Vintage speaker. It's half the weight of regular speakers as it's made of neodynium (spelling?), and seems like a good new option.
Let us know what you decide, speakers contribute so much to the tone of an amp, yet few people seem to consider changing them.
Cheers,
Jordan.