I'm a huge fan of Drone and Doom....and Drone Doom
For me it is the layered tones, the atmosphere, the density more than anything. Don't check out live performances for an understanding, look at some released songs.
Sunn O))) are, as you noted, one of the more popular ones, but check out the following:
"It took the night to believe" is probably one of my fave Sunn O))) tracks.
Earth were the primary influence for Sunn O)) (some may say that Sunn O))) are basically just an Earth tribute band of sorts...
), but they took a drastic turn towards non-distorted drone a few years back, and
i love it. I often use the track
"An Inquest Concerning Teeth" for prayer, actually! Very relaxing and contemplative I feel!
For more generic doom (without the drone influence) you can't do better than
Electric Wizard. Much more 'normal' and closer to mainstream metal (you could easily mistake it for Sabbath). Try
Funeralopolis by Electric Wizard.
For some other really good black-influenced Drone Doom, try
Country Doctor by
Burning Witch or
Skin Coat by
Khanate
Sorry for overdoing it with the Southern Lord bands (Southern are a pretty prolific Drone and Doom label), but I think if you listen to all of those and still don't get it, there's not a lot more I could do to explain it!
All of those songs are available in their album-recorded format on YouTube. I would have put the links in myself, but apparently the forum won't let me until I have more posts!!