Warning: preg_replace(): The /e modifier is deprecated, use preg_replace_callback instead in ..../includes/class_bbcode.php on line 2958 *sigh*...Opeth
I personally like the whole double-kick-drum/growled vocals thing, especially the sudden changes to it that Opeth use. But I know what you mean, it's not to everyone's taste and it can definitely get boring after a while.
They're not strictly the same genre as Opeth, but you might want to check out Mastodon (if you haven't already). Particularly the Crack The Skye album, and the songs: