C or B, that's the lowest you can go with 22/24 fret guitar.. After that you need to start extending neck to get intonation right and you lose more attack the lower you go. Not to mention that B is ~60Hz, pretty much everything below that don't form decent chords specially with heavy distortion. I have drop B tuning (B-F#-B-E-A-C#) on same looking sunburst Fender strato '80 and i've already run out of room fix intonation (thank goodness for my Rockinger Tremolo bridge circa '81 and it's flexibility..)

I dunno. Mastodon seem to manage ok on standard neck length down to A# or even A.

I didn't find this riff "heavy" at all.
It's not the tuning that makes a song heavy, it's a whole bunch of things.

Sepulturas Roots Bloody Roots is in B and Panteras 5 Minutes Alone is in D. Can you honestly say that Roots is that much heavier? Would 5 Minutes Alone be heavier in B?

The riff itself is obviously important, but also what the drums are doing, the production, the tone, etc.

You are right, what i was saying is mostly about audio quality. You can not have a sharp fast complex attack on very low notes. No matter what the instrument is. B is about the lowest, after that you need to make drastic equalizations to get that sharpness back. Also the chord structure "widens" it's intervals the lower you go. With B tune, you can barely make fifths. Once octave higher and you get thirds. If you go 7 octaves higher, you can even use seconds... Allthou Bmin9 sounds AWESOME on B, not very pretty or even recognizable, it's just rumble..... But it sounds clearer around E, aka Breath on Floyds Dark Side of the Moon....