well in a sense you answered the question already because there is a divice to get that crunch sound and it's called a distortion box,or overdrive box.
Typically it has to do with clipping the signal and or overdriving the tubes to get that sound.Personally I have alwaysprefered to get my crunch from the amp rather than a box.Though I'm thinking about getting a Ibanez tube screamer to get that balls to wall Stevie Ray Vaugnn blues crunch for that type of music.
I use a Mesa Triple Rectifier which has incredible crunch but nice clean also.I think Marshall amps get a warmer smoother distortion that I actually prefere in the classic rock type playing but the Mesa handles many types of songs all for me.
Here are some recents vids I did to study myself on flaws (got a few lol timing,bending on and on)and also use for want band ad I have.I need to get a pitch control CD player though because the one I'm using brings the pitch of the track a little higher and it's hard to tune to exactly match it.The vid speaker recording doesn't do justice to the amp sound,it sounds better in person .
http://www.city-data.com/forum/other...y-time-my.html
Lowering the tuning on a guitar can add low to crunch also,like Drop D tuning like for instance what the song "Higher" by Creed is tuned to.Most metal guitarists drop their tuning and use heavy gauge strings because of this.
Guitar,amp,pickups and different effects pedals deliver different distortion sounds also.
