That sounds like it is working as intended. The "loudness" feature is only to be used in a calibrated system where Media Center is the only thing used for volume control.
You cannot change the amplifier's volume or it invalidates the calibration.
You have to set things so that the speakers measure 83dB when using Media Center's built-in test signals. (
Tools → Advanced Tools → Audio Calibration)
Whatever volume level Media Center is at when the system measures 83dB, is what you set as the reference volume.
Then as you adjust the volume control below that level in Media Center,
the loudness curve is adjusted to compensate for the reduced volume.
In theory, this means that audio should sound the same at low levels as it does at high levels.
If you have volume set at 30%, and the reference level at 30%, there will be no compensation applied, as it should be playing back at 83dB.
If you change the reference level to 75%, with volume still set at 30%, you're now telling Media Center that the playback level is 60.5dB, so the relevant loudness curve adjustment will be made.