IMHO the reason you want to use an amp is that it colors the sound of the guitar. PA amp is supposed to be linear/flat response. so it doen't provide the necessary eq/cliiping etc.
OTOH, i used the setup like guitar -> BD-2 -> computer sound card, it sounded quite decent. the BD-2 (a mild driver) acts as a preamp to slightly color the sound of the guitar, and provide some needed overdriven sound. just watch the level, normal speaker are not designed to be overdrived, you will get hard clipping and blown speaker if you don't keep the level in check..
A fun thing i tried: get a pair of $19.00 portable passive speaker, plug into the headphone out of the amp, and turn the amp all the way up. you can actually hear some nice speaker distortion at very low volume level.