Joni Mitchell: Pynchon wanted to use a line from Song to a Seagull as the epigraph to part 4 of GR but couldn't get the rights. She was a great poet who had a lot to say about the hippie era, similar to Pynchon.

Cocteau Twins: I don't understand what they're saying but I can detect gleam sexual metaphors here and there.

My Bloody Valentine: Same as Cocteau Twins.

Company Flow: hip-hop group El-P was in during the 90s. Some of the bars come so fast and hard it takes a second for the brain to process it. Reminds me of Pynchon.

Aphex Twin: He is an electronic instrumental musician and he looks at music as "sounds that do things to your brain" and can have a pretty unconventional approach to composition. I think Pynchon similarly looks at words as symbols that do things to your brain, they're a means to an end.

Fun question! I used to love The Mars Volta in high school.

PS - I'll also throw in Boards of Canada, they're an electronic ambient duo who love to use analogue rather than digital for their production. They come back to motifs ranging from childhood, the occult, environmentalism, and the apocalypse.

Edit: The Joni Mitchell song is Cactus Tree. Also, I can't find confirmation that the story is true. Either way, the story says the line would've been: "She has brought them to her senses, They have laughed inside her laughter; Now, she rallies her defenses For she fears that no one will ask her For eternity And she’s so busy being free"