A label I haven't seen mentioned yet, is the German "Brain" label.
This label specialized in prog, electronic, Kraut rock, jazz rock and related genres. Records on this label were universally dead quiet, dynamic, clear and detailed.
Some artists on this label are:
Neu! - offshoot of Kraftwerk
Guru Guru - kind of a free jazz rock band with Zappa, Pink Floyd influences
Grobschnitt - early on, some of the best space rock around, later a bit more prog. Always had a lot of humor in their music.
Klaus Schulz - one of the best electronic artists, ever.
Release Music Orchestra - world class fusion. With some similarities to Weather Report
Embryo - eclectic, energetic, jazz rock with some world music
Novalis - classic symphonic prog rock with King Crimson influence and some folk
Schicke, Führs & Fröhling
. Prog, jazz, avant-garde. 2 of the members, Fuhrs and Frohling recorded an album called "Amerland" with some haunting and beautiful music.
Another thing to look for, is not specifically releases on the Brain label, but almost any recording by Engineer, Conny Plank, in his studio.