I wanted to smartassedly point the user to concrete subreddits that would fit, but are there any, really?

/r/AskHistorians is great perhaps even better than HN in signal/noise ratio but it only has "news" on Wednesdays (and that feature is not very popular).

/r/AskScience has a similar problem and /r/Science is getting too many jokes/memes/regurgitated press releases.

I don't even know where to start for music, /r/WeAreTheMusicMakers/ ?

For music, the place for discussion OF music (rather than about making music) is /r/letstalkmusic, which is actually one of the highest quality discussion subs I know of. It's got a relatively slow but incredibly high quality stream of discussions, of all aspects of music history, various bands, philosophy and psychology surrounding music, etc.

For music discovery, /r/listentothis is alright (although it has declined greatly since becoming a default). /r/listentous is sometimes much better and sometimes much worse, by the nature of the sub (elected recommenders). The real best places for discovery are the genre-specific subs however, i.e. /r/metal, /r/futurebeats, etc.

Sub reddit discovery isn't a solved problem but the subs you're looking for probably exist.