Thanks for the link. Yes, I made sure I fully understood the pros and cons of using adopted storage before deciding to try it. It's not an issue for me, because I have unlimited 4G and therefore never download anything. I've only ever streamed.
It's also even less of a problem for me now, because, after four years, I've cancelled my subscription and moved over to Google Play Music (sorry!). I like the way it lets you disable caching whilst streaming (and I'm hoping for a YouTube Red subscription at no extra cost when they finally launch it in the UK!). Although, to be fair, you can't download offline playlists to adopted storage on Google Play Music either - it only 'sees' your 'real' internal stoarge - again, not a problem for me as I never download anything.