I promise I'll write something of my own soon, but now Marcus Scott (whom I never would have suspected was a devotee of this sort of thing) enters the fray, with some errata and addenda:
Got to remember too, earth 2, which is kind of the ur lp for a certain type of doom was out on uber grunge label sub-pop, not a metal label, so to an extent it's the messy misery of grunge taken to extremes, the guitar sound on earth 2 isn't as punishing or heavy as the guitars on the sunn drone tracks or something like boris when they've done drone. I reckon it chimes in a timely way with what koner etc were all up to.
Southern lord is a metal label too, O'malley himself was a journalist for the black metal scene etc who did a magazine called descent, he knows his shit obviously, a lot of the other bands on the label are metal proper. Incidentally do you know about Probot, a heavy metal supergroup, who released on southern lord, featuring members of (black metal originals) venom, lemmy from motorhead, lee dorian from cathedral etc... all singing over tracks written by - erm David Grohl from Nirvana/Foo fighters, so in a way this stuff's roots are the splits between grunge and metal, grunge being one of the last times I think guitar music had much to say in terms of a popular music form.