I guess the thing(s) that made me think they are enhancing their voice was (1) a lot of those vocals sound the same to me, (2) with both of those bands (Korn, Slipknot) there are portions with milder vocals and they're actually kind of whiny, nasal little voices (same guy?)

I've listed to Cannibal Corpse and it's not for me :) Not interested in death metal anyway but the band sounded alright. I'd never be able to listen to vocals like that.

I like singers like Matt Barlow, formerly of Iced Earth. I don't mind a little bit of the gutteral singing, just not ALL of the time. Too bad he won't be singing in a band anymore, or so he says.