Hi
I believe the AUX input in the unit i bought is done through the quadlock connector pins 7 & 8 in the image below (just my speculation). As it can't be using the same circuit as the original car AUX connector because from there there is definite signal degradation.
I did conduct an experiment with 320kbps mp3 file.
Transfer it in android unit - good sound.
Stream it with Bluetooth - good sound (changed Bluetooth codec settings in phone developer options to LDAC).
Burn the same file into CD disk and play with navigation unit - good sound. I did try some other store bought CD-s that had pretty much the same overall sound grade.
I would say they all sounded equal.
Now i did not have anything connected to the car AUX input.
I discovered you can have one of the 1...8 buttons set as AUX input (in my case set 8 as Aux input).
1...7 are set as radio stations.
While android interface was open (and music playing) i pressed 1 and radio started playing while the actual display remained on the Android side. I find this awesome where you can listen to radio while kids could play some games on it with Playstation Dualshock controller (haven't tested with PS controller it but then again see no reason why it wouldn't work).
Then i connected phone through the car AUX port and played the same 320kbps mp3 file which sounded awful. Also tried some old ipod which was also bad. There is clear distinction to be heard. Almost like highs and lows are cut off and is very quiet on same volume. I'm thinking should get rid of this now obsolete and quite useless AUX hole and put something useful there like the double USBs.
Might pull everything apart for the USB experiment this weekend to see if it uses pins 7 & 8.
Hope this information is helpful.