In this case I don't think we can level the blame on HDMI (for a change). From what we have gleaned by collective experience, it is to do with how well the TVs implement the surround sound decoding.
It seems that with the "multichannel" setting (and a HiDef output) the Humax passes on or recreates an encoded audio stream (Dolby? AAC?), which (it seems) sends the volume as data so that the waveform data can be encoded at full resolution (and therefore use fewer data bits per channel). This might have been a change to the specification.
If the surround sound decoder in the TV (regardless of whether it has surround sound, it still has to decode stereo out of the surround sound data stream) only decodes the waveform data and fails to recognise the volume data packets, sending multichannel audio to the TV will result in a constant volume signal.
This is all hypothesis you understand. In any case, as we have said many times before, for best quality it is better to send the sound at max level from the source and control the level at the final point (ie use the TV volume control, not the Humax).
(Edited in the light of post 21)