1. If you plan on seating people in multiple rows and truly care about what your audience hears, you should get multiple subs. I suggest you get 3 EP350s instead of 2 EP500s unless you feel you can afford 3 EP500s. The subs should be placed at the mid-points of walls or in the corners. The corners may lead to bass heaviness so you will have to experiment. If you don't want to experiment, place them at the mid-points.
2. Do NOT place your M80s into cabinets if you plan on doing any music listening (particularly 2.1). You will totally destroy their off-axis characteristics causing them to lose transparency and present a smaller sound-stage. Some may tell you that you can put them in a cabinet as long as they protrude a little. I say no way. The cabinet you put them in will colour their sound.
3. You will need to experiment with this. If you don't want to experiment, angle the QS8s down so that they are 45 degrees to the normal.
4. They will augment the sound stage but the question is should you get QS8s or direct radiators? Again, you should experiment. If you don't want to experiment, go with direct radiators. Place the rears 1/3 of the distance from each side wall.
5. You'll have no problem whatsoever with that Onkyo and the M80s.
Good luck and post pics and reviews when done.
