I got a GLC350 loaner car which is is equipped with MBUX and I have to say that it is an audio abomination! First, it is still afflicted with the ever present and totally annoying speed controlled volume pestilence which cannot be turned off. And they have made the gain versus volume curve even more aggressive just to rub some salt in the wound. I don't know if forcing this menace onto customers is born out of ignorance or arrogance on the part of MB. And just to add to that, some inexperienced junior grade engineer made the decision that each of the discrete volume steps be at least 6 db each! So your only choices are too soft or too loud.
After driving with this system for a couple of days, I found it so obnoxious that I just parked the loaner car and drove one of my other cars on which I have been able to solve the SVC problem. I have been able to defeat the SVC problem on all of my previous MBs, but the lack of a DVD player and the high degree of integration of the MBUX system means that will not be possible with MBUX nor will it be possible to replace it with a properly designed audio system. Indeed, this issue could spell the end of a new MB in my garage after 35 years.