I'm doing a lot of research on this as well (probably the same as many) and what is needed is a simple "MOST" interface from the input of the fibre optic feed to the HK 'amp' module.
If one can be found and made to work with the head unit & software of the '5, then it's output could and would become so much easier to make use of.
As it stands right now, as you say, any amp used (and being 9 channels its a few of them) needs to take a high level input.
This is where I am for now and waiting on some further details from them. Their distributor is not so far away from me thankfully.
https://www.audison.eu/products/bit-dmi/