I have Standard audio system and had been unhappy with the "quality" I was getting out of Slacker (my primary source) and standard radio. However, when I was on HD Radio, the quality was miles ahead of Slacker. I did have Slacker set to "best" in the car, but it was nothing compared to HD Radio. Today I used Slacker's website to enable 320 Kbps on my car's Slacker account and I have a significant improvement, particularly in bass response. If you can get the credentials to turn on 320 Kbps support (and have a strong enough 3G connection to support it), do it. The difference is very noticeable, even on my standard audio system.
To me, this ultimately means that Tesla's sound system hardware is capable, but the sound processing on lower quality sources is very poor. This could be why I previously complained that Tesla's audio system was lacking (particularly in bass), but was very clear. The sound processing was probably inhibiting bass if it didn't meet certain quality "depth" levels. In my 4Runner, I had significantly more bass (compared to Slacker pre-320 Kbps), but much less clarity and some distortion in the music.
Honestly, certain bass heavy songs were able to vibrate my mirrors at 7, must less needing to go to 11! And this is without an aftermarket sub!