Are you suggesting this applies only to voice calls being in stereo? I know when I stream pandora via bluetooth on my '13 its definitely in stereo.Gen 1 Bluetooth do NOT support stereo streaming, mono voice calls only.
I've done audio through USB accidentally when plugging my android phone in to USB, which I didn't expect or like. Ended up having to remove all of the music files from my phone since it would automatically start playing every time I connected the phone.
Regarding bluetooth and at the very least Pandora yields reasonably expected audio quality. I don't really notice much loss in quality but then again I don't think Pandora reaches audiophile quality to begin with. Certainly no discernible hissing popping or artifacts over using the 3.5mm connection.