There’s a pull request in the Raspian GitHub to add multichannel PCM output (192k/24bit/8ch) to the ALSA driver in the Linux kernel:
github.com/raspberrypi/linux/pull/1166
More discussion on the topic about the HW capabilities and different implementation approaches here: github.com/raspberrypi/linux/issues/1000
Would love to see this change rolled into the next Volumio release.