Problem :
Compressor/Expander does not seem to kick in as prescribed by its threshold parameter
Explanation : Compressor threshold parameter is in fact a threshold per channel. In unlinked mode this has no implication as each channel is processed independently. In linked mode, the Compressor/Expander FX consider the average of the power in all channels to and compare this value to the threshold. The fact that threshold parameter is per channel then become useful as a parameter set can be re-used from a mono sound to a stereo sound (and eventually multi-channel) which has more power and preserve the perceived behavior of the algorithm.
When the mono/stereo SFX is routed to a bus where Compressor/Expander FX are inserted, the plug-in receives the number of channels specified as the Speaker Configuration (e.g. 5.1). These 5.1 channels are fed by constant power 2D/3D panning/positioning algorithms which will steer the power contained in the mono/stereo SFX to the various output channels. The resultant of these algorithms is that there will be as much power but located into (possibly) more speakers. Because the Compressor/Expander inserted afterwards on those 5.1 channel busses use the average threshold over 6 channels and that power is most likely only present in a subset of those channels, the threshold parameter may need to be reduced to get the compressor to kick in (opposite situation for expander).
