Actually, it is possible to coax a decent bass sound out of a guitar amp. It requires the proper equalizing, at least 100 watts, and not too much gain.
Stone Spoon BaboonCheck out the track Slinky 3000 on the above link, we recorded that with my drummers 120 watt Line 6 Spider guitar amp, in a live band situation with drums and using one mic. My tone on there is pretty decent, though a little more overdriven than I'd have liked for that situation. Obviously it's not the best situation for bass to go through a guitar amp, but in a pinch something like that can perform quite well.