Activating the THD Boost function raises the level in front of the gain control element. This generates additional harmonics inside the input stage of this amplifier, which has a significant influence on the sonic structure of a signal.
The mpressor 500 is a pure feed forward compressor, and the gain potentiometer is placed before the control element. This means that the intensity of the saturation can be raised or lowered by different settings of the gain controller.
What’s special: The detector in the sidechain of the compressor runs in parallel and is therefore not influenced by the THD Boost at all. The actual compression does not change, it is only the sound which is being influenced.
You generate a saturated signal which as a result is also reduced in dynamics, but the actual control of the compressor itself is always based on the original dynamics including all transients, impulses and so on.