Several readers, including Oliver Reeve, have reported distorted sound from their iSub when using GarageBand. Reeve writes: "I have just installed GarageBand on my flat panel iMac 700MHz with 768MB RAM. When I play tracks on it the iSub distorts badly. In fact if I play regular tracks on iTunes while GarageBand is open the iSub will distort as well. If I make a track on GarageBand then export it to iTunes and close GarageBand the tune plays distortion free in iTunes. It makes no difference if I plug the iSub into a USB hub or directly into the iMac. The only way to stop the distortion is to mute the iSub or turn the iSub output right down."
A thread on the Apple discussion boards contains posts from several other readers having this problem. Most posts indicate that the distortion occurs mostly at mid-range frequencies, with extremely low bass coming through relatively clear.
One user reports that going to GarageBand's preferences (the Audio/MIDI pane) and changing the "Optimize" setting to "Better performance". Some readers report that this method removes some or all of the distortion, while others report little change in the sound quality after switching the setting.