Your speakers need to be calibrated to an 83dB reference level and all volume control handled by Media Center for this feature to work correctly.
If you don't have a calibrated volume level, I recommend that you do not enable the Loudness feature.
I did do that.
Even if I didn't, it shouldn't matter. The equal loudness contour is supposed to be a large boost in the lower frequencies and a smaller boost in the high frequencies at lower levels. Therefore, as you increase the volume by X, the midrange increases by X, but the lower and high frequencies increases by an amount less than X.
What JRiver does is if you increase the volume by X, the lowest of the lower frequencies increases by X, but the midrange increases by an amount greater than X. While this method can achieve the same sonic experience as the above, the SPL on the volume slider in JRiver does not match the output SPL.
Again, if you don't believe me, turn on loudness, go to DSP studio's analyzer, throw a pink noise on, then increase the volume by 10dB on the JRiver's volume slider. You'll see on the analyzer that the midband is clearly increased by more than 10dB.