I don't think you'd be able to blow the speakers if you tried. They're very low power, and they probably have some sort of limit for what you can put in to them. As far as your original question, I personally had been using my guitar, into a distortion pedal, into my powerbook. That works well enough, but it still lacks a little punch. I tried running it directly out of my amp head (crate blue voodoo), and didn't get much better results. I think the best option is going to be either putting a mic in front of your amp in some sort of reasonably acoustically sound enclosure (like a box with a bunch of carpet). I'm going to try this over the weekend. Also, I'm interested in trying to use some sort of multieffects pedal with amp modeling, such as the newest Vox things (which actually have a tube preamp), or some sort of line 6 POD. Other than that, I don't think you can adjust the settings on the included settings in garageband. I could be wrong, or perhaps you could with the 100 dollar upgrade, but I'm not about to find out. It really bothers me too, because my friend and I have had great success with recording drums and vocals, but the guitars just don't have the great punch and crunch that they do when we're jamming away. I'll have to upload one of our newest creations so you can get an idea of what is possible.