if you want to use another compressor, patch it from:
'Pre VCA Insert Send', and return 'Post VCA Return'. (this will remove the quad+ master fader out of the signal path)
You can re-purpose the comp and master fader for use elsewhere on the console, leaving the patch points:
'Pre VCA Insert return' (and the balanced input to the quad) and 'Post VCA insert send' (as the output of the quad)