Mandrake, welcome. You'll find the QSs to be magical for side surrounds. As to the back surrounds, it may be a relatively close decision. Using QSs there also would be advisable if you were sitting relatively close(say 4-5')to the back speakers, which might make the direct speakers a little too direct at that short distance. On the hand, if you're much farther away(unlikely with a 15' room)the directional speakers would have more room to disperse and could still locate effects specifically intended for the back a little more definitely.

-----------------------------------

Enjoy the music, not the equipment.