Is caching YouTube videos for offline use illegal? What about iPhone apps like Music Tube?

So there's this album that I Really want to get, but it is not available on Spotify. I tried looking on places like Rdio, Beats Music, Google Play Music, and pretty much every music service like Spotify available in the US but I couldn't find it. I even looked for it on iTunes, but I couldn't find it there either. However, I did find the album on a version of iTunes from another country but I can't get it there since I live in the US, and I can only buy stuff from the US version of iTunes. Recently, I discovered that there are some apps where you can cache YouTube videos for offline use, and the album that i want is on YouTube. These apps use YouTube as a music player. You don't download the videos as MP3's like all those YouTube download stuff you find on the internet. You cache them for offline use, like how you'd cache Spotify tracks to listen to offline. These are the apps I'm talking about: http://appcrawlr.com/ios-apps/best-apps-cache-videos I want to use them but I'm afraid they might be illegal, and i don't want to risk breaking the law when I get songs you can't find on iTunes or any other streaming service, or unreleased songs by my favorite artists. If these apps are illegal, can you give me some legal ways I can get songs I can't find on any music services like iTunes or Spotify, or any other music service available in the US?

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