The HD621 does not do any decoding on it’s own (Dolby Digital, DTS, etc…). It accepts LPCM over HDMI, extracts that signal, and passes it on via 3x SPDIF or MMHR on RJ45. For 3x SPDIF, the maximum is 6 channels, so 5.1. The MMHR connector can output 8 channels, so 7.1 is possible.

MMHR is only available on the 861v8. For the 861v4, you’re limited to using the MD1 or MD2 inputs, which would be 5.1.

Cheers

1. Trinnov Alt32, Meadowlark Semi-custom 4-way Digital mains, Custom 3-Way Digital center, M6, DSP420, HSU VTF2
2. G68D, DSP6000, DSP5kC, DSP420