hey Nick,
that HVAC and drainage pipe may give you lot's of grief in terms of unwanted noise in your HT area. Is there a way to put a wall there?
I would get 7.1 if you're willing to spend that much. Just remember, most DVD's now are only 5.1, some are 6.1 but are generally matrixing the last rear speaker.
If you listen mostly to movies, get two pairs of QS8's for the back. If you listen to DVD audio or other 5 channel audio, get a second pair of mains for the back.
Your area is not huge so it's a budget thing. If you can afford, go 7.1. If you can't 5 or 6.1 will be more than enough. I used to have a space about that size and filled it really nicely with M22 mains, QS4 rears, VP!50 center and EP125. Bass lacked a little but then I boosted to an EP350 and problem solved. My receiver's an inexpensive Denon 1602 with a modest 70wpc into 8ohms.
What you're buying far tops that
good luck, and happy listening
