Agree with Davisjl - found that whatever Sirius/XM receiver they're using is garbage - on most channels you can hear how lossy compression is, which is unfortunate because I love the Sirius/XM music selection. Using CDs, or an iPod via USB, or Spofity/Pandora via bluetooth on your telephone, or a good HD radio station produces much better results. Heck even the Sirius/XM app on my phone sounds way better then the onboard receiver...
I also found I had to set my fader back to almost halfway between the back and neutral/balanced position, crank the mids and set the bass and treble to approx. 3 bars (it's tough to tell because it seems like each bar has like 3 or 4 positions before the next bar lights up or disapears) to get optimal sound from the system in all 4 seats. Took me like 45 minutes or tweaking settings and switiching seating positions or so to get it all dialed in. Now I think it sounds pretty good. It's not the best system I've ever heard, but in my humble opinion it does sound better then the base MKZ system - I drove a base MKZ rental on a 5 day business trip - so I did have some basis for comparison.
Just wish they would use a better Sirius/XM receiver.