I have a little valve amp, i think its 25watts, and whilst i like the way it sounds, its a little on the quiet side. i find that i have to turn it to practically full volume for gigs and it gets really distorted. whilst i kind of like this sometimes it would be nice to be able to get a decent clean sound. could i just buy a cabinet and put i through that for more volume?
cheers

You could try adding a line-out and run it to a power amp, PA, what-have-you. The link below has some suggestions.

http://www.rru.com/~meo/Guitar/Amps/Kalamazoo/Mods/ezlo.html

-sam

you can even with many DI boxes take the signal from the speaker itself and attenuate it down to a level suitable for the PA. Otherwist micing or adding a line out would seem more sensible. Sennheiser e609 great for live micing without too much spillage.

If you dont want it going through the PA at all, consider trying to get hold of a nice transparent sounding keyboard amp with 10-12" speakers and stick some of the preamp output through that. I did that as a kid and made my lovely little amp fill some mighty big spaces.

If you were to buy a cab with lower impedance speakers to increase volume you would place an abnormal load on the amp and most likely cause it some damage, im not so sure about guitar amps, but PA amps definitely dont like odd impedances!

You might want to find an amp tech who could build you a "slave" box, or even put a slave-out jack on your amp (though if it has vintage value, you might want to go with the box). Some guitar amplifiers have a slave-out jack on them, which is a tap on both the preamp and poweramp. You can run your main guitar amplifier through a separate power amp and speaker cabinet to make your main amp louder.