If someone thinks Black Sabbath, Toxik, Suffocation, and Hibria ALL SOUND ALIKE, they have something wrong with them. Hell, going from Suffocation to a band like Possessed is still a noticeable difference.
Also I really hate the "Metalheads only listen to metal" thing. While some do, that clearly isn't representative of the whole of the genres fans. Just look here:
ZackP1992â€™s Music Profile â€“ Users at Last.fm