What you need to do is get a lone of a good 60band + RTA and measure the speaker lead output of each unit with pink noise. To see if its VW added frequency responce shapping to one or both of the units.
I wonder if the RCD510 is similar to the RCD310 for the frequency responce or more like the RNS510?
Can you describe what your hearing?
Does it by any chance sound like a reduction of frequency level at 200Hz Yes and below as you turn it up and maybe a dip in the responce at ~5Khz? Yes
Some answers in red.
Others below:
(1) The RNS does not seem to produce as much bass as the RCD. The RCD goes deeper and bass drums can be felt.
(2) There seems to be better attack with classical music with the RCD.
(3) The RCD seems to have cleaner high frequencies which seem to extend further. Triangles and cymbals sound cleaner and clearer.
(4) The RCD seems to sound cleaner when loud.
(5) The stereo effect seems greater with the RCD.
(6) The soundstage has great width with the RCD.
I think I can get as close to the RCD sound fiddling with the tone controls, though they can only do so much.
It seems the fader can improve things by moving it more towards the front loudspeakers.
It is pretty good now and I am reasonably happy with the way it sounds.