With HDMI the downstream component tells the source component what its capabilites are. Thus, if you set the HD621 to 2-ch it will tell the player to output 2-ch only. Thus on LPCM you only get a downmixed stereo which is then being processed by the G61. This is why you get PLMovie as the preset automatically selected.

You should set the HD621 to 6-ch. This will then allow the full data to be transmitted to the G61 for processing finally being output as 2-ch to your speakers.

For Bitstream, the HD621 just passes this through to the G61 for processing. As this feed contains all the music data (just compressed) it will sound better.

Malcolm