Finally, the new amplifier board arrived today. An Adafruit Max9744. 

Put it into place immediately, and it works perfectly. 

Audio input is coming from the Sid Tap Headers, as described in my prevoius update. This means that the audio is not processed by any FPGA components on the mainboard and directly fed into the amplifier  

I still have to find a good solution for the volume control.

I‘ll probably try to print a better looking knob and will also print some frame to fix it to the enclosure.

Whatever it will be, you‘ll be the second to know.

Summarizing: audio is playing perfectly fine, no hum or other disturbing noises.