When my GTI was new, I took a spectrum analyzer and measured the response of the sound system with treble/mid/bass levels at '0'. The system is somewhat OK. It exhibited a flat response from 100Hz to 15kHz which is OK. However, it is comparable to average boombox sound quality. I have heard better boomboxes.
Sound is subjective so I cannot tell you which setting works best for you. However, to achieve the right setting, you need your music source to output a flat (no boost or cut tone adjustments) response. Then from there, work the treble/mid/bass settings on the radio. The trouble with a lot of music sources is that bass/treble/mids were adjusted during recording instead of recording them at a flat setting. If you start with a music source like that then it is best to keep the radio settings flat so you do not introduce unwanted increase or decrease of the treble/mid/bass. This is recommended if you were the one that introduced the EQ to the music source during recording. If you just got your music from the net then there is no way to tell if the music material is recorded with EQ or none. What you think is a good setting on your radio's treble/mid/bass for one music track may not work well with your other music tracks.
Another reason why the sound system may sound crappy is when you are used to listening to over-boosted music. This is a good way to corrupt the perception of good sound. I won't argue with you if this is what 'floats your boat'.
Does the Premium 8 Sound System in our GTI's need more low-end bass, yes. Does it need better tweeters, yes. I also find the mids lacking probably because the door speakers are too low in reference to ear height. Do I wish for a better sound system, yes. However, I no longer spend too much time in my car so I could not justify spending money on better sound system components. I have not heard the Dynaudio System so I could not comment on how good/bad it is. Some swear by it. Some hate it.