The problem is not the quality of the mp3s themselves (they are a mix of 256 and 320). It is the system in some fashion. It is not a small difference in quality - it is massive - far bigger than the difference between 128 and 320 bit rates. With the eq in neutral, the mp3s sound as if the singer has been put in a box and the bass booms (from the door speakers) - very unpleasant to listen to at volume.
I am less convinced now that it is just a case of SD card vs Bluetooth though. I use Poweramp and Google Play Music on my phone. With the app eq set to neutral, Google Play Music is similar to the SD card but with maybe a bit less bass. Poweramp is noticeably crisper when set to neutral. But with both, by using the EQ, you can easily get some decent settings (with Dynaudio all set to neutral).
I also compared 256 mp3 vs 192 wma - they were basically identical in the car - you would need a much better listening environment than a moving car to tell them apart anyway.
There is something wrong, at least with some systems and media sources. Maybe it is just default choices they made about how to set things up, but when combined with the useless EQ facility, you cannot correct for it when playing from SD card. My guess is that my favourite genre, rock music, is far from what they tuned it for, and also they did not tune it to be neutral. Given they put a sound maker in the car (which I had disabled before delivery), maybe they decided Golf R = poser = massive bass, which unfortunately means rubbish sound quality...
I do notice that when playing DAB and listening to other music, it seems much more suitable - crisp and the bass works better, though 80s stations show the same problem when playing Queen etc.
Brenbo - I guess you have an older version of Dynaudio, set up quite differently to mine (by the factory). It is a shame you are in Cornwall!
In case it is relevant, the part numbers are:
Device part number: 3Q0035842A
Hardware: H24
Software: 0245
Media Codec: 2101.0.0.10