Grindy/grainy sound comes from how hard certain frequencies are clipping. When mids and high mids are being pushed really hard in the preamp, they clip to look more and more like a square wave (instead of a sine wave, which would be a clean frequency). Your brain perceives this single "square-like" wave as a combination of many individual sine waves: fundamental frequency plus LOTS of higher odd order harmonics... graininess.
MDV is correct in that the amp has a lot to do with that, but how hard you are pushing the amp in the first place has a lot to do with it too! Higher output pickups (especially EMG) really have that grainy/grindy character as a result of the type of signal they send to the amp (i.e. how hard they push it in certain frequencies).
I'm not sure I answered your original question about the Aftermath tho lol. Shoot Tim an email.
EDIT: any frequency getting clipped really hard will distort to a square wave, but the mids and high-mids are the frequency ranges our ears hear best and give a sound its character (imo)
