the key aspect of the heavy is a sense of movement. thats why the baseline for the kate bush song counts. when it starts you feel a physical sense of something dropping. the god flesh song feels like theres a little factory in your chest, a piston turning slowly over and over.
humming slab
then you get the totally grooveless heavyness of noise. really most of it is shite but merzbow is the don. which is in most situations the most aggravating sound in the world. unignorable. you can't play it anywhere or in any situation except sitting by yourself in the right mood. and half an hour is plenty. excessive even. there is a way in though about one in ten times that i try, its definitely the kind of thing where you have to pay attention, and then its absorbing, you follow the flows of static up and down, its not dissimilar to 'difficult' jazz in respect of how you listen to it, and then there's a nice effect sometimes where you turn it off and anything else you try to play sounds wet, and you end up noticing the sound of the boiler or the air conditioner or whatever, after getting absorbed in this essentially hostile sound
Sleep were the kings of heavy for a number of years before doom became a thing. "Jerusalem" is from '98.
There are different textures of heaviness too. You take something like the above or that Fall tune and there's a grit and growl to it that emphasises the weight but then you can also get a rounded, spongey sort of heaviness, e.g. Hokusai - 'Black Rose'.