As non-native speaker I think I'd intuitively go with “Metallica is” and “Nirvana is”, but “The Beatles are”. I guess my brain is more wired towards whether the name itself is plural or not.
What about "Dire Straits"? Also plural, but grammatically completely different from "The Beatles" (by which I mean: you could say that John Lennon was a Beatle, but you can't refer to Mark Knopfler as a Dire Strait)
Dire Straits is something that happens to you, it would be used in this context, 'He was in dire straits' meaning, in big trouble of some kind. The straits in question are not a plural of an individual living thing like a Beatle. So I would say 'Dire Straits is a band'. I'm a native British English speaker, that sounds right to me, but my generation weren't formally taught grammar at school so I can't back that up with any fancy grammar words!
