Firstly 'old hard rock bands' like Type O Negative and My Dying Bride... who I remember when they were new... that's a big old  

 from me...  

I'm a metal fan, but this does seem to be a very niche genre.  Metal has so many genres and sub-genres, part of what I love about it.  I did 10 minutes of research and had heard of almost none of the bands mentioned.  To make matters worse, it seems that 'doom metal' is split into eight different sub-genres, according to the hardcore fans!
Black Sabbath seems to be the overwhelming influence over this whole scene.  I like Sabbath, without actually loving them.
I know that Wikipedia isn't usually regarded as the most accurate of resource materials, but it may help...
https://en.wikipedia.org/wiki/Doom_metalBe interested to see what finer metal-minds than mine think...