I'm not sure anyone has but..you can now get an Audison ASD bypass harness in the UK for around £15 saving time n money over the Technic PNP version. The two harnesses are functionally identical.
BMW purportedly has spent over 180m Euros over a 10 year period working both internally and with several universities on accoustic deisgn in cars.
So the ASD is quite a clever box in how it works, in the same way as lossy encoding, accoustic cancellation and perceptions of humans about the 'quality' of sound varies depending on the enviroment they are in..
I don't know whether the top end 677 audio unit amp's DSP has been tailored tos upport the ASD, but the ASD itself has its own DSP and maps for generating engine related sounds.
I would imagine that the 677 amp frequency response / audio response curve is tailored to suit so you are not as likely to see a marked effect compared to say fitting better speakers / DSP /amps with what in isolation would be the most appropriate response curve with ASD.
Most DSP/AMPs for cars have a marked bass uplift to suit the specific nature of a car cabin..ie when the controls are 'flat' there is a still a maked gradient, fairly linear when viewed on a log scale with bass boost and treble cut..
The ASD is very bassy so any system that plays to improved bass response either through better speakers or audio /accoustic profile will over emphasise the ASD sounds..
Mine was very very noticeable after my mega admp/dsp/speaker upgrade..
All IMHO..