Is there a music player that can play music from the Google Music Store as well as Amazon MP3? It's getting really annoying that the music I "buy" in the different music stores on Android (Google Play and Amazon MP3 specifically) aren't visible to other music players even when I make them available offline.

Are there any players out there that know to dig into these areas and make this music playable? It's really obnoxious to not have all my music in one place and have certain tracks tied to players I don't especially like.

The only solution I've come up with is to abandon getting new music when I'm out and about and download the tracks on a PC the old fashioned way and then add them to my music folder on my phone by hand. Surely there's something better that I haven't thought of yet.
Have you tried CloudAround yet?  It's a music app that lets you listen to everything on your device as well as sync/stream from a few cloud storage providers.  This includes offline music you saved from Amazon MP3.  The app scans your device for all music files and gives you a handy list of everything available.  Once you login with the various cloud services, your music stored will show up too.