The opening act(s) arent going to matter in the big picture for drawing. Usually the exception is the all day shows or big package tours.
Heaven and Hell have hit most markets 3-4 times in the past couple years, thats likely why its drawing less.
8/11/13 Baroness/Royal thunder Royale Boston
8/13 Baroness/Royal Thunder Met Cafe/Prov
7/16 Mayhem Fest Mansfield (maybe - so so lineup)