I had a bunch of them 15-20 years ago. Different ones were more or less complex, and went in different places in the system. The basic idea, as I remember dimly after all this time, was that higher frequencies tend to travel through cable more quickly than lower ones, and consequently can cause an artificially bright electronic sound. What these did, as I recall, was slow the highs in relation to the mids and the mids in relationship to the lows, on some sort of adjustable slope (internal resistors, I guess). The problem was that they weren't cheap, but worked most effectively on cheaper systems. When you put more money into cables and electronics, you made them largely extraneous, and the very people who would pay for such tweaks didn't need them. I'm sure there is more theory to it than this capsule description, but this was my experience.
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