I am starting to think that tone control circuitry and potentiometers have traditionally not been of very good quality. This may be the reason that they have been avoided. I also think an industry arose to accomplish the same ends. In many ways tone control has been replaced in various ways by adding levels of complexity to make a system "synergize." I will be the first to plead guilty to having done this. I rolled tubes, made many cabling switches, even more fundamentally I have bought different speakers, amps, preamps, phono stages etc.to achieve a system I like. I think we all can that different systems do in fact sound different.
That said in defense of those who think tone control is anathema, I must agree it can be dreadful. For instance I tried a mid fi parametric equalizer from the late 70s or early 80s and it absolutely flattened the soundstage, muddied the sound and simply made a good system sound terrible. The possibilty of introducing useful tone control arose when using a computer based equalization program popped up and demanded to know what type of sound I preferred. After trying numerous presets I made a custom scheme. I found it improved the sound (obviously a matter of my taste vs the 0+- line.)
With current advances in understanding the target audience, I now think it is realistic that hardware engineered for critical listening proximal to the source with digital data clearly, can compensate for recordings that are not well made or you just want to sound the way you like it. As mentioned if Leben can achieve high quality with bass contour AKA "loudness button" then amplification circuitry by others should be able to as well.
