I was fighting in my sleep last night if the soundstage would be too close and not get good separation when objects move across screen. In that case I would just get m22s and mount the on the walls around 9ft apart or so. Although I really want the m80s
First if your listening position is 6-7 feet (from your screen I presume) and your speakers are 6 feet apart you should exceed or be at the generally recommended equilateral triangle distance. If that turns out to not be wide enough is there any reason you couldn’t put the M80s in front of the AV cabinet and the sub? If accessing the cabinet is an issue turn it to face in.
To your original question I’ve been using QS8 stands exclusively and find they work great with two exceptions. They’re top heavy so unless they are against a wall I’d weight them down. Also, I’ve found that when using high backed chairs mounting the QS8s up a couple of feet about ear level works better.
I also agree with Randy (sirquack) that you might benefit from a single surround back speakers on the stair wall. Because you don’t have a symmetrical back wall to help enhance the rear surround field adding a single surround back might be worth it. While a 3rd QS8 might work best I think even a single M2 would might be a more cost effective option.