i think that burtons vocals from Fear Factory had a big impact, how many nu metal bands rely on the "gruff acidic (they think it is, dont burst their bubbles) vocals then harmony" style of singing, linkin park, papa roach, whoever you want to name. this style of vocals became most obvious in Demanufacture, that came out a little before the numetal exsplosion in the 90s
Well none of them were actively involved, but if wasn't for the specific elements they had Korn and nu metal wouldn't exist. There were tons of bands in the late 80's/early 90's who Korn stole ideas off. FNM, RATM, NIN and Nirvana are just the main four. Soungarden, Red Hot Chili Peppers...the list goes on.