Crossover (for the purpose of a battle-of-the-bands) with Detroit Metal City, which is a send-up of Japanese black metal/death metal culture. Soundtrack is from Tenacious D and the Pick of Destiny, as I recall.

One of Yui's delusions from the first episode included Johannes Krauser II, the front man for DMC's titular band... which is the record-company-mandated alter ego of Soichi Negishi, a farm boy from Oita prefecture (think, like, rural Iowa), and Detroit Metal City's protagonist. In the live-action film version, this is what he looked like when not in-character as Krauser:

(I picked that image because it better communicates the contrast between Negishi and Krauser than any of the manga panels or anime stills I found in a quick Google image search.)

Detroit Metal City is a comedy, despite what the video above might suggest -- Negishi aspires to record insipid Swedish-style pop¹ -- Hōkago Tea Time's usual output has a harder edge than he's comfortable with, just to give you an idea -- but his paying gig is ... well, most of the really disturbing imagery in the AMV I linked upthread comes from a Detroit Metal City music video produced in-universe.

1. That's the anime's ending theme, and, in-universe, one of Negishi's signature songs.