When you have the Xbox Music Pass and download the songs, they still have copy protection and need to be decrypted by an official device.
If you want DRM-free MP3s that can play on any device, you still have to buy them with real money via the Xbox Music store. The subscription only allows you to "rent" the songs. They are still DRM-protected and will no longer work once the subscription ends.
The good thing is Xbox Music has a lot of sales. Like they had full albums for free or for only 99 cents for an entire album. And if you get the Xbox Music Deals app, you can get notified of deals on albums for $6.99 and below.