I own a Classic 30 and just recently ordered a Keeley MT-2 (upgrade from the stock). I havn't received the pedal yet, but I was wondering if I wanted to further improve my tone, should I buy new speakers or tubes, as in which would make a larger difference?

I was thinking of buying a Celestion Vintage 30 speaker, not sure about the tubes yet. Im trying to get a sound suited for metal, but more importantly a fat, crunchy tone for solos?
i'd say do the speaker...i've seen people use tubes, but they really really really know what they're dealing with.

"so this guy woke up with 6 different girls on 6 days of the week?"
"yea, isn't that crazy?"
"he must be god"
"...and on the seventh day he rested"
how often do you have to change the tubes, in normal circumstances
Tubes should get changed everyone 1-2 years based on how much you play anyway. You could get some higher gain tubes if that is what you want. I think tubes would make a bigger difference than a speaker.
get some new tubes. it will be cheaper. also, many times amp companies ship thier amps biased slightly cold so have an amp tech rebias your amp so its "hotter"