Bear McCreary's soundtracks to "Battlestar Galactica" are quite good. Actually, I started watching the show after seeing him perform live. Some great moody, dark suites in his body of work.
E.S. Postuhumus is wonderful as well. You'll likely find pieces that have been used in every movie trailer ever. Some of it more uptempo than you're looking for, but the slower stuff really pulsates.
Brian Tyler's Soundtrack to "Children of Dune" has a track called "Inara Sharif", where he recreated the language from Herbert's books to make a choral section. The instrumental stuff is top notch on there too.
GSYBE, Burial, might be out of your tonal range but always worth mentioning. East hastings by GSYBE is pretty marvelous.
Amiina is also good, I've got one album of theirs Kurr, and it's very very good.
There's also Kangaroo Moon, which is kind of like psychadelic electronic folk. It's quite good.
There's free jazz too, if your mood swings that way, most of the stuff I have has vocals though, although it's more like barks and grunts and howls, so you might still be able to consider it instrumental.
Alexandre Desplat's soundtrack to Jonathan Glazer's Birth is moody and beautiful. It's one of my favourite soundtracks ever. It's a bit hard to track down, but it's worth the effort.
ETA: Actually, here it is. Some of it is a peppier tempo than you may be looking for, but there's some very minimalist slow stuff, a mixture of orchestral instrumentation and electronic mooing. It makes sense when you hear it. Watch the movie, too. It's hugely underrated.
I would suggest nest which is acoustic pastoral/pagan folk and bohren & der club of gore which is very slow* moody jazz (like blade runner only slower, minimaler and cooler).
Any Howard Shore soundtrack for Cronenberg, especially Crash. Philip Glass? No-one's mentioned Glass yet? (I especially love the 'qatsi s/tracks and Candyman. I understand he also did some opera stuff...) Dead Can Dance. Avoid the bits where Brendan Perry sings, lose yourself in it when Lisa Gerrard sings.
The Fountain soundtrack. That's Clint Mansell and Mogwai, working with the Kronos Quartet. Biggest track on it, Death is the road to Awe, is a twelve-minute tantric orgasm. And if you're actually doing tantric orgasms at the time... well, words don't do justice.
Most of what I listen to are soundtracks, when I want instrumental works. Top films besides the above - Peter Gabriel's Last Temptation of Christ, James Horner's Name of the Rose, Tangerine Dream's Near Dark and Sorcerer, Kenji Kawai's Ghost in the shell 1&2, Tom Twyker's Perfume and Run Lola Run, Michael Nyman's Gattica (his least clangy score!). And I have a deep love for the best Batman score ever - Shirley Walker's Mask of the Phantasm.
Personally, Boards of Canada just give me a headache...