If it's a bass guitar amp, you might get away with it. You
will find that the snare, hats, and cymbols will sound as if they have been
muffled with a sock but the kick will be okay - maybe.
If it's a normal guitar amp - DON'T DO IT! Guitars have a
completely different sound to drums, very much in the mid-band range. The 12"
'bin' or speaker as the rest of the world calls it will not cope with the
masssive transitient signals you'll throw at it. The driver may well end up
distrotying it self.
As for your Jamo's - you may well get a reasonable sound out
of them, not so much a Hi-Fi speaker, more a disco speaker for home
use.
Using them live might be pushing you luck, but you may well
get away with it.
Goldern rule is, if it sounds as if the speaker is about to
distory it's self - it probalbly will!
A good keyboard amp (min 150W) is more the sort of thing you
want to be looking for. 500W would be plently loud enough.
Have a 1000 W + PA gig - Better still!
the Sonic Energy Authority - a sound
investment