The problem was with the digital outputs: the processor was attenuating by 3dB, to give some headroom for the multichannel presets Trifield etc., but not required for a 2.0 or 2.x setup.

Version 6 or higher firmware fixed this issue.

861v6+ID40, HD621, 8000.2, 7200HC, 7200.2, SKYQ Silver, PS3, PDP-LX6090, QNAP TS870-Pro (8*6TB), Roon, iPadAir2.
[In Reserve] C15, MD600 (2*3TB), TwinStore RM (2*2TB)