To my ears, even Morbid Angel and Possessed are closer to being "proto-underground" than, say, the first Incantation album. That is the start of death metal. Though there generally seems to be a lot less rigid classification of genres in the early days, which allowed for a wider array of influences. Not becoming too narrow-sighted (not necessarily a bad thing) or inversely adopting any and all influences just for the hell of it.