@all: Thanks for your comments!
Just to clear up the confusion around my chain: Currently the miniDSP DDRC-24 is analog tied in between the preamp and power amp. Volume control and source selection is via my preamp.
The HPA is connected via rec out of the preamp with a fixed level.
My first attempts to replace the preamp with the DDRC-24 failed miserably, as I could not detect level or source.
In addition, it is unusual for me to control the volume for the HPA via the minidsp without a screen.
I expect much more comfort from the miniDSP Flex.
Another consideration is to buy a separate DAC for the headphones and use the HPA separately only for the streamer.
Do you think I will have any sonic losses if I replace the elaborately designed nuControl with such a "little digital box"? My gut feeling still resists a little against this step.