Loudness works with predetermined crossover frequencies and Q-factors.

The variable values you mention are from the bass and treble control, a different function, without connection to Loudness.

To setup Loudness function:

•  If necessary, set Bass- and Treble control and the Equalizer until the sound is to your liking.
• Determine your usual normal listening level's volume setting.
• Set the Loudness' "Low Vol Ref" 20-25dB below this level.
• Now adjust Loudness' Bass and Treble Gain and fine-tune the "Low Vol Ref", until louder and quieter Vol. settings sound about equal in the Bass/Mid/Treble- balance.

With these settings the Loudness function is out of the way with normal listening levels and chimes in when you need it, compensating apparent loss of bass and treble when listening at lower levels.

To disagree with Curt (that's the first time!!), for me the Low Vol Ref is 20 dB under the normal listening volume, not the maximum volume setting...

Thanks for the input. As I already like the loudness function it is not a big loss that it is not adjustable and will take your word for it that it is not worth the effort to dig into.

I have however investigated the purpose of the loudness function, which is the reason for my question. The ISO 226:2003 provides nice equal loudness curves, very similar to fletcher-munson. Correcting for loudness would then be the difference between the the equal loudness curves. My finding was that the curves suggest rather little adjustment above 500 Hz as the curves are largely parallel. Below 500 Hz this seems not to be the case and I wondered how much was to be gained compensating in the 100-500 Hz range as well as below 100 Hz.