The Tact RoomEQ gear after the 1st gen included volume specific correction for loudness. It worked great. You even could custom alter the response changes with volume. Haven't seen anything matching that automatically.
The reason they went away is usually they weren't right except for one volume and wrong for all others.
In my opinion, they went away due to the pressure from reviewers. Around the time tone bypass/line direct buttons really kicked in. Having a loudness button was seen as not HiFi enough for the audiophiles.
I have absolutely no problem with loudness contours- when bypassed, they are out of the circuit and if they work for you that's great.
Not all loudness contours are coupled to a 50% loudness tap, but they all should be. You can have >15dB boost on many, and at high volume positions (where it's not needed anyway) that would spell blown woofers. The few manufacturers that implemented "bass" or "low level" contours that were independent of volume pot rotation used considerably milder boost to prevent over driving at high volumes.
Sansui and others interlocked the loudness and tone controls so only the tone or the loudness could be used together. Most others were not so clever.
Audyssey is a step in the right direction but it is still a Volume pot controlled solution. In these days and ages, it is possible to implement a correct, configurable Loudness control. . More than doable, have a mike always plugged in, evaluating the SPL in question and adjusting the EQ according to Fletcher-Munson equal loudness curves and perform the necessary corrections.... Bang & Olufsen (the brand so derided by audiophiles but whose speakers will mop the floor with many audiophiels darlings) seem to have such a solution...
Asking permission to rant and digress a bit? Ok? Granted? Thanks
Let me begin.
Someone has posted that one of the rasons that these buttons went away was that they weren't "audiophile enough" , I agree. The entire hobby was/is awashed with an ersatz of "purist" mentality . It is easy to point its contradictions, it has nonetheless develop a philosophy wrapped around the notion that things must be analog and the signal untouched by the deleterious manipulations and calculations of the "mechanical" (!!) digital . Tone controls on a system are perceived as inferior .. better use cables, components "synergy" to modify the sound, or better limited bandwidth components such as full range drivers and SET to deliver pure unadulterated sound (such a "dream" system would be an idler pulley turntable a SET DHT amplifier of 4 watts or less driving a pair of "full range" drivers that can't play a sound at frequencies under 150 Hz or above 10 KHz ..
) or cables with mystical powers or ... Science be damned !!
Back to the scheduled discussion.