Iron Butterfly's the big one here, but I already know about them.

I'm looking for something more rooted in the heavy end of the psychedelic rock spectrum/acid rock, and preferably something more drugged-out than, say, Blue Cheer.

It has to be from the 60's?

yeah, you would have better chance if the stuff is from the 70's. You should check out the 2 first Alice Cooper Group records...

http://www.youtube.com/watch?v=fxfeCxCDXSs

http://www.youtube.com/watch?feature=en ... nFscLiP3CY

I think I can help you out here although some of it is from the early 70s and not just the 60s.

For the past few years I have become OBSESSED with exploring the very earliest roots of metal, doom metal, proto metal and downer rock.

First of all I would recommend you pick up the "Downer Rock Genocide Compilation" if you can find it.

It includes the following artists who were some of the earliest proto metal bands that many have never heard of:

The Original Iron Maiden (previously known as BUM)---This was an English band called Iron Maiden who existed BEFORE the NWOBHM band, they formed as BUM in 1964, changed names to Iron Maiden in 1968, released their only album MAIDEN VOYAGE in 1970 and broke up.

Very cool old school proto Doom.

Flying Hat Band--Had Glenn Tipton from Judas Priest
Writing on the Wall- Interesting Scottish Proto Doom/Pscyhedelic band
Red Dirt--Heavy blues
Slowbone-Heavy Blues
Necromandus--had ties to Sabbath, I think maybe the same producer as some of the early Sabbath albums
Clear Blue Sky
Monument--obscure early proto doom
Iron Claw--excellent early proto doom band that has falsely been called Antrobus in the past for some reason
Eegor
Gnidrilog--Weird early Heavy Prog
B.U.M.
Bram Stoker

(Also includes Hackensack but I think they are just typical hard rock)

Other early Proto Doom/Proto Metal/Heavy Pscyhedelic bands you will want to check out:

Coven--(ONLY the first album) Witchcraft Destroys Minds and Reaps Souls (after that for their other 2 albums they oddly went Country Western...)
Bloodrock--Especially the first two albums and the song "D.O.A."
Warpig---I CANNOT STRESS THIS BAND ENOUGH!! Early undiscovered Canadian Doom!!
Bedemon--HAS MEMBERS OF PENTAGRAM!! Child of Darkness--Released in 1974
Zior--has members of Monument and cover art done by the same guy who did the art work for Sabbath's self titled
Flower Traveling Band--great Japanese heavy acid rock
Warhorse
Atomic Rooster

Shit, I KNOW I know of a lot of other early bands like this but this list will have to do for now.

I'll think of more later.

Give Sir Lord Baltimore a try, great acid rock with lots of bluesy influences but also
that badass crazy vibe proto metal bands had

I would also recommend Captain Beyond.

There aren't many of these kinds of bands from the 60's, and I know you are familiar with The Jimi Hendrix Experience, Cream, Led Zeppelin, Blue Cheer and MC5. Not heavier than Blue Cheer, but still heavy for their time were The Jeff Beck Group, some people say Arthur Brown, but I didn't give him a listen yet. You must have heard of The Stooges, but I still had to mention them, that guitar was so heavy for 1969. Proper proto-heavy metal from the 60's would be The Gun from 1968.

http://www.youtube.com/watch?v=kFyEjUXGE2I

As someone in the comments wrote it sounds like a mix of Black Sabbath and Cream. Another proto-heavy metal band that released an album in 1969 was High Tide, just listen to this.

http://www.youtube.com/watch?v=16SocVdK4wE

There is this weird band I always bring up in these kinds of threads is Cromagnon.

http://www.youtube.com/watch?v=rPiO_G-DEHs

Some people claim they sound like some proto-pagan black metal band. I've read a couple of opinions that said that 21st Century Schizoid Man by King Crimson, was the first heavy metal song. Those will be all the examples that I can think of that I have on my disc, but I know a load of bands from the 70's which match the description.

I don't care if these are from 1979, 1980, and 1981. They've got a proto-metal sound, with lots of space rock, prog rock, and psychedelic influences on these albums.

Check out
Manilla Road - After Midnight
Manilla Road - Invasion
Manilla Road - Mark Of The Beast