I've only had the 4311 for a couple months now, so I'm not terribly familiar with it. It sounds like you may have dynamic volume enabled. If you do, disable it - it is evil. Dynamic EQ is great, but not the volume. Try that first. There is also a setting for reference offset, but I don't really know what the hell that does.

On your Opppo, make sure you have HDMI and bit-stream selected. Make sure you have the Ohm setting in the 4311 set to 8, not 6 or 4 as logic would suggest.