Also of note, Selected Ambient Works Volume II. I used to mix ambient tapes from a variety of sources in the mid-90s, and this was always features somewhere. One good mix was Autechre's _VLetrmx21_ over Aphex's _Blur_. None of it is in sync, some of it's dissonant, but it works.
I miss 90s Ambient and IDM. Em:t label of was a top one. There was a sound to all of it, and it probably had to do with 99% being produced with outboard gear straight to tape. Everything now seems to be so over-produced and "perfect".