Loudness is for listening at significantly lower level then usual.
It's meant to keep the perceived bass/midrange/treble balance as independent from listening level as possible.

The procedure below works the other way round than described in the manual page 15.

I assume that at "normal" and slighly lower listening level an unchanged, "No Loudness" sound is prefered.

STEP 1, basic setup:
• Determin your "normal" listening level's dB or dBr (*1) value.
This may vary slightly depending on the source and music.
Estimate an average "normal" dB value.
• Do all settings needed for this to sound best, EQ, Bass, Treble ...
• Set the Loudness "Low Vol Ref" 25 dB below this "normal" listening dB value.
This way Loudness correction kicks in 5 dB below normal listening level, leaving all above untouched.

STEP 2, fine tuning:
• Now dial down to your usual "low" listening level.
•  Adjust Bass and Treble Gain to an amount that the bass/midrange/treble balance sounds comparable to what you hear at your "normal" level.
Go back and forth beween levels to check.

To give you figures:
My "normal" listening level: -30 dBr (with Auto Reference Level: On)
Loudness Bass Gain: 9.5dB
Loudness Treble Gain: 7.5dB
Loudness Low Vol Ref: -55 dB (Loudness slowly kicks in at -35 dBr)

With different sensitive headphones, speaker setups, different loud sources or music styles, "normal" listening dB value has to be re-determined and "Low Vol Ref" adjusted accordingly.

Finally to mention:
Loudness is NOT the right tool to correct transducer flaws.

(*1) Remark:
Volume in dBr instead of straight dB shows up if Auto Reference Level is “On”.