Personally I distinguish a drone sound made by extra long reverberation from a drone made with a sustained tone.
Japan has a tradition of reverbered drone music, Chihei Hatakeyama's style is very similar to the artists you mentioned. Calibrate long tail reverbs is way more difficult than how it seems. For this kind of drone music I really enjoy Rafael Anton Irisarri who's oceanic sound is massive and soft at the same time (https://www.youtube.com/watch?v=bfLImkV_l6E)
Maybe you say "hardcore" referring to the distorted sound of bands like Sunn O))), which is a style I've never been into...
But I guess by saying "hardcore" you want to mean "radical". Sustained tones drone music sounds more austere and explores the resonances produced by the combination of different sustained tones. The austere sound is often the result of notes that are not tuned in the standard tuning that dominates western music (notes of the keyboard) but tuned with microtonal settings that sounds bad at first listens, in order to produce more harmonics.
Check out this outstanding performance where we can hear a piano that resonate way more than normal
That being said, sustained tones drone music starts being interesting when you start focusing on the harmonics and the creative sum of tones, which can produce a wide range of emotive transport, from magnificence to softness. Plus you start notice how much movement there is in what seems so static during the first minutes of exposure.
I find super interesting the use of acoustic instruments as source for long tones, from cello to organ, but recently many new sonorities have been reach by the use of digital softwares such as Pure Data or Super Collider where you can have no limits in the tuning features. http://isakedberg.com/Λ.wav
Bonus suggestion: https://www.youtube.com/watch?v=AX8LONLCTJ0