Hello Markus,
The attached circuit may fill your needs. It takes advantage of a recent addition to the ADAU1701's toolbox, the log-spaced lookup table. This greatly simplifies dynamic controls like this one, by providing a numerical index that varies with the log of its input (a dB-sensitive table). There has always been a base-2 log function built into the ADAU1701, now we can finally get at it.
All else that's needed is to set up two General Second-Order Index Selectable filters, one for bass and the other for treble. Varying level chooses different filters, as shown in the Real-Time Display. Built-in slewing renders the transitions inaudible, as long as the filters are closely-spaced. I used ten filters plus flat. Of course you can add more filters and adjust them in line with published curves; I just made it sound good to me. Wish my stereo had one of these.
Best regards,
Bob