I like to filter my 3 main group buses (Drums & Bass, Keys and Guitars, Vocals) first with an EQ so that each bus has it's own space in the mix. Then I'd use a MS EQ like the Pro-Q 3 on the master bus. I find that if you fix your sub buses first, than you'll have a much better result with your master bus. I'll also add a correlation meter like Voxengo Span as my last plugin to ensure mono compatibility.