As TLS writes the loudness button can work pretty bad for reasons the pro audio manufacturer RME explains in the quote. For me a good loudness implementation is useful because I generally listen at lower volumes, though sometimes I turn it off, depending on content.
Audyssey room EQ has Dynamic Loudness that generally works well but is based upon you first having done a calibration and is configurable to a certain degree with setting a Reference Level Offset form 0 dB. So bass/treble is only added when below RLO.
The RME DACs have a loudness feature that also works well and is quite configurable to taste and circumstances. The following is from the manual for the RME ADI-2 DAC FS:
>>>Another legacy of HiFi amplifiers: there has not been a single one missing a feature called Loud-
ness. It tries to address the changes in frequency-dependent hearing sensitivity over different
volume levels. If one listens to music loud, then drops the level by at least 20 dB, sound loses
punch and glitter. HiFi amps tried to fight this effect by adding more bass and treble the lower the
volume was set. Unfortunately that never worked as intended, and just became an additional
bass/treble booster. Reason: the manufacturer of the HiFi amp could not know what volume any
position of the volume knob equals at the customer’s home. Room size, room dampening and
efficiency of the used speakers are all unknown.
But the effect of loss in perceived sound exists (read about the Fletcher-Munson curves), and can
be easily reproduced with any serious gear by comparing normal volume and DIM state (usually
-20 dB). The ADI-2 DAC offers Loudness for the analog stereo outputs, and probably is the first
time that Loudness works as intended. The user can decide how much maximum gain in Bass
and Treble should occur at lower volume settings. The user also sets the Low Vol Reference,
where maximum gain is achieved. After extensive tests a 20 dB range has been defined as range
for maximum gain to no gain while increasing volume. That seemed to be the perfect definition of
the range that needs to be addressed by Loudness. <<<