Definitely add a 2nd sub. Of course, assuming you feel you aren't getting enough bass. Some rooms do really well with one sub, maybe you got lucky. But adding a second sub to our room, which is quite a bit smaller than yours was a major upgrade. I'd also find a way to get those surrounds closer to ear level. I have my qs8's up close to ceiling as well, and they suck. Yup, after years or ignoring the issue, I finally admit it. It's not the speakers, but position. So I'm moving mine to ear level but my room is challenging as well. Since I added M3 ceilings for Atmos, it's just painfully obvious that ceiling mounted QS8's are the wrong way to go.
Besides, getting them lower will make them louder, and easier on your amp too.
