I haven't used garage band, so I don't know exactly what it can or can't do. A lowpass might help, but I think a lot of it is from the sound source. What are you using to record the guitars? IMO it's best to try to get a good signal from that start because trying to tweak it afterwords is hard. Usually harsh guitars come from too much top end(no mids) or too much distortion. These days there are tons of cheap options that will give you a decent recording sound, but you have to tweak them. BTW what type of sound are you going for?