Low pitches allow a DEEPER sound. This usually adds up to "heavier" though.

I have my 7's down a wholetone, to A, which without the 7th is D.

I have to admit as well, you get high tuners who are heavy as shit, listen to the palm muting at the end of Hammer Smashed Face. E motherfucking b!

But a lot of the time, high tuners are thrashing the strings at great speed and backed by a drum beat for their heaviness. Slayer (Eb) for example. Low tuners, like Meshuggah on their 8-strings ( low F), or Nile (drop A), are pushing out immense heaviness off the back of single notes.

So I guess it's the bass that determines heaviness. Listen to the Hammer Smashed Face chugging. They wreak of bass.

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