you can, but it will sound like shit. won't get the thumpy tone. maybe it's ok for a while, but after a long time the speaker will tear. unless you're a brainiac, try to avoid slapping real hard.. electronics of the amp will remain fine though.
i came across a guy who had been playing bass on a guitar amp for many years. appearently he wasn't aware of it till the day i saw his amp. :lol: the amp was still fine. only the speaker sounds a little broken. btw, his amp is 35w, so it's powerful enough to deliver the bass sound quite decently.