The HC is a special circuit I have worked on for many years, which is designed to restore a certain tube-amp essence of tone to modelers and preamps, with the goal of making the modeler sound more believable and amplike than it actually is. The HC not only improves the tone, but makes the overall "feel" & "playability" of the modeler better, while greatly reducing the amount of time required to program pro-sounding patches. It also makes many more of the Amp, Cab & OD models in your modeler usable (maybe for the first time!) The name "Harmonic Converger" was not pulled out of thin air - it's actually very descriptive of what the unit does *sonically*.
Have you ever noticed that many distortion tones have a bogus-sounding, "mosquito-esk" hi end, that is seemingly imported from another universe, and sounds totally foreign to the main body of the tone? Well, the HC sonically "converges" (homogenizes) the sound back into a cohesive/satisfying/expressive tone, which sounds very organic to the human ear. Part of this process eliminates unnatural "fizz", but more importantly, it adds the brightness & timbre of a good tube amp, imparting a special tube-esk "urgency" and "snarl" to the otherwise boring midrange frequencies. Because of these improvements, the low end also sounds tighter, and can be boosted substantially without muddiness. Even great tube amps don't reach their fizz-free, growling sweet spot until the level is painful, but the HC does it at *any* level.