The "Back to Mono" post by @russ69 is a reference to Phil Spector, who in the early-70’s had a batch of buttons made with that phrase emblazoned on them. There are pictures of Phil with John Lennon during the recordings of John’s Rock ’n’ Roll album in which Phil is wearing one.
The singer/songwriter/organist leader of the Garage Band known as The Lyres---whose given name is Jeff Conolly---went by the name Monoman. I have a bunch of their albums (on LP and CD), and saw them live in the late-80’s at Club Lingerie on Sunset Blvd. One of the most exciting live bands I’ve ever seen. The sound system at the venue was a mono one. 