I experience phenomenal sound from my early Eonon GA6166 in my factory Nav, non-DSP Hi Fi system.
Here are my volume settings which work out just fine for me:
I simply used the long Fakra harness cable from the head unit to the FM tuner cable under the spare tire, aft of the battery.
E53s with non-factory Nav make the same connection to the factory harness up front and do not have to drag and bury that long a$$ cable all the way to the rear.
My hypothesis for the OP is that something else is wrong, perhaps in the head unit, especially if the tech connected the HU to the factory harness properly.
Mike