Multichannel plugins do not function reliably in the Master Section. When inserting a multichannel plugin into the Master Section, an error is usually produced, “Error reported by L360 5.1 - At this insertion point, the plug-in cannot accept the number of input channels.”
I have tested this functionality with the latest versions of VST3 plugins by Waves, FabFilter, Steinberg, and others. All of the tested plugins operate successfully in Steinberg’s Cubase but fail in WaveLab, suggesting a general problem with the Master Section’s handling of multichannel plugins.
This issue was discussed in Steinberg’s forums (see here) but I wanted to rephrase it as an official bug report, rather than a support request. [Unfortunately, there’s no “issue” tag in the WaveLab forums, so it’s unclear this is serving any purpose. I hope it’s helpful.]