RAAT multichannel over HDMI, Linux - anyone?

I’m trying to find best way to use my spare Cubox as second, multichannel only RAAT.

Yesterday, without any problems I managed to output multichannel flac from my Intel Nuc (Windows 10), over HDMI, to my Marantz NR1607.

I tried this with Cubox, Armbian as OS. Unfortunatelly when I choose in Roon setting HDMI output there is only 2.0 speakers setup (greyed out).
Anyone can give me any tips how to achieve my goal?