Thanks HQ for the informative post! It's great to see this sort of interaction and information flow.
Future Firmware
We also have an upcoming firmware release for 8x8 to enable DSP speakers to mute when using the analogue out to make integration of the Prime Headphone Amplifier even easier.
This is very interesting, and it's really heartening to see added value for headphone users spreading from the Prime outwards to other components!
About that 8x8 firmware, are you able to offer dither appropriate to headphone listening? (Minimum Audible Pressure curve, as detailed on p. 17 of the 518 User Guide.)
And for me a HRTF/ASP-equivalent in the digital domain would be a really big deal, as I already have a full headphone setup. I'm sure that would make the 8x8 even more attractive to other headphone listeners too.