RPI, just buy a $10 USB stick and leave your tunes on that. iDrive will display them in whatever folder format you choose to store them in.
I only wish there were 2 USB ports in the car, so I could use both a USB stick and my iphone5 at the same time. I wonder if a USB splitter would work...will have to try that.
I've read that XM sound quality is roughly equivalent to 32kbps (1/10 the data rate of a CD)...which is crap.