I use my PC and MAc both for looping. I have Cubase 4 installed on them both and it's a very useful tool. I've played around with Garage Band some, in fact it's got some good loops already in the program, but mostly I use Cubase.
I've done some midi and audio looping. I'm not sure about garage band but with Cubase you can use the snap tool and cut your audio recording on a preset of either beat (whole, half, quarter, sixteenth...note) or you can set it to bar. This really helps out when creating your loops BUT you will need to record your loop with a click for this.
As for bass percussive sounds, thats a great idea. I usually, in fact 99% of the time, record my bass lines as a whole. I like to add slightly different touches here or there. But percussion, rhyhtm guitar tracks, key board..I loop em.
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