Sorry I don't speak German, but I understand 80% of it 
I am from Belgien and have my GolfV since Friday last week (now 390km)
I have the rcd 300 radio and the el cheapo speakers but I also read, and heard that these suck major time 
I've been reading Vadder's page which graph says enough (4 a 5KHz peaks!!!) Which makes this car sound like a telephone!
You can also notice phase distortion when using the fader from rear to 3/4 front, it seems like the sound is sucked away from you at 3/ front.
Now I started to look into this and it seems that the filter from tweater (ht) is not correct, it should start at at least 9KHz.
A change of capacitor, smaller one , or adding one should make a big change!
Now this is a lease car and I don't want to dismantle the door! so I went the cheap way for now.
What I did ... I took some 'invisible' tape and covered the tweaters (ht) and then made 3 small holes in the tape.
The tape and holes make a high pass filter.
I now have to turn up the treble but it sounds much much nicer!