Already in 1950, Peter Baxandall designed an analog tone correction circuit which found its way into some million consumer audio devices later on. Today, it is simply referred to as a Baxandall EQ.
What the f*ck is a Baxandall EQ?
Beside its appearance in numerous guitar amplifiers and effects, it made a very prominent reincarnation in the pro audio gear world in 2010 with the Dangerous Music Bax EQ. The concept shines with its very broad curves and gentle slopes which are all about transparancy and so it came to no surprise that this made it into lots of mastering rigs right away.
And it also had a reason that already in 2011 I did an authentic 1:1 emulation of the very same curves within the Baxter EQ plugin but just adding a dual channel M/S layout to better fit the mastering duties. For maximum accuracy and transparancy it already featured oversampling and double-precision filter calculations to that time and it is still one of my personal all time favourite EQs.
During the last 10 years quite a number of devices emerged each showing its very own interpretation of the Baxandall EQ whether thats in hard or software and this was highly anticipated especially in the mastering domain.
A highly deserved revival aka renaissance.
When comparing units be aware that the frequency labeling is not standardized and different frequencies might be declared while giving you same/similar curves. More plots and infos can be found here (german language).
Variety Of Sound 
Nice article and references. I tend to use the Baxter EQ on the mix bus of my mixes – and sometimes also on individual channels (like for instance on the lead vocal for high end shimmer). It is a very smooth tool to work with.
Hello there! I downloaded the new 64-bit version of the Baxter EQ and the GUI doesn’t show up in Reaper. I haven’t tested other daws but just wanted to point that out. I would love to use this EQ for mastering and hope to get it sorted out.