Extension cabinets

[MartinBarre]

What exactly do they do?

If I have a 20W amp, and I add a cabinet rated at 150W presumably the cabinet won't be pumping out the full 150W it'll just match the 20W the amp can provide?

So does this effectively DOUBLE the volume of a combo amp?

I'm looking at cabinets for my Marshall AVT 20 (which I love) and they're £180!
Would I be better off just selling my amp and buying a 50 or 100W amp, rather than adding a cabinet that's costing more than my amp did!?

[Spivonious]

Well, first of all, the cabinet rating is just the maximum watts the speakers can handle; it has no effect on loudness.

In general, more speakers = louder maximum sound, since you're moving more air. BUT, you'll also need more power to drive two speakers as opposed to one. So in the end it enables you to saturate the amp a bit more to get an equal volume. If you like the sound of your 20W but need some more presence on stage, get the cabinet. If you aren't happy with your amp, adding more speakers is not going to make a difference.

[Ace]

Also the Valvestate amps have a soild state power amp, so the lower the impedance the more power your amp will be able to push through it. A 4 ohm cab would work best for you my friend!