Loudness is based on
ISO 226:2003 equal loudness contours, and requires a fully calibrated system, which Media Center has volume control over.
So when Media Center's internal volume control is set to 100%, your system's volume level (measured via an SPL meter) must match the reference level of 83dB.
As you reduce the volume in Media Center, the equal loudness contours are applied so that the perceived frequency response does not change due to the reduced volume.
When calibrated with a reference of 83 dB at 100% volume, if you play this back at -20 dB (60% volume) the frequency response should closely match the 60 phon contour for example.
Adaptive Volume set to night mode applies dynamic range compression, which can make things like dialog easier to distinguish at lower volume levels, because the difference between the loudest and quietest sounds is much smaller. It will also reduce the volume of sudden loud noises like gunshots.
I don't think it would be advisable to combine the two.