No luck finding an earlier example of Eno comparing ambient and metal but no less than four people emailed to say that they recalled a Gibby Haynes from the Butthole Surfers quote about listening to metal ambiently:

"I'm listening to quite a lot of heavy metal these days. It seems like metal's got so fast, it's slowed down. If you play it at a low volume, it's like ambient music, like a chorus of buzz-saws."

One correspondent, Steven Stamper, managed to track down the source on the web, and it turned out to be an old MM interview with the Buttholes by yours truly. Circa 1990, so musically they'd gone right off the boil by that point, but still superior quote machines to 99 percent of rock bands then and now.

That Jack Officers record, the concept sounds so promising--Buttholes meets acid house... god it was awful...