Reading between the lines of that it's a known issue which B&O consider to be Mercedes fault
Is the bass actually present despite the noise/distortion? It sounds like a bad enclosure volume and or port - ports can chuff horrible and under damped woffers can bottom out giving the horrible flapping sort of noise
If it comes to it I know a guy who is an acoustic consultant by profession and a petrol head and audiophile by choice. He certainly has the equipment required for a very detailed analysis and a professional report
By the way, congratulations on an otherwise fabulous car!