To elaborate a bit, the input to the quad comp is "PRE VCA Insert Returns", and the output of it is one set over on the "POST VCA Insert Sends". As noted above, when you make those patches, you will be breaking the normals on the quad bus and will no longer hear it. You can get around that by patching directly from "Pre VCA Insert Sends" to "Post VCA Insert Returns". Now you've jumped the quad bus across the compressor, and can still monitor the quad bus in the center section and access the normal quad bus mix outputs as usual. As noted above, the master fader goes with the bus compressor, so you won't be able to use it on the quad bus in this scenario.