Beth,
While I agree that the EZ set anchors are one heck of a device, I would be hesitant to use them unless you are certain that the sheet rock on the ceiling is 5/8” and the joists are on 16” centers. All it will take is one accidental bump to the speaker and the paper on the rock will rip, which will ultimately lead to the speakers dropping at the most inappropriate time (most likely with you under it). Another concern is if there is foot traffic above the mounting location. Vibrations from walking above the speaker will eventually cause the rock to weaken.
Sheet rock gets its strength from the paper that binds it together. If the paper tears, the strength is nullified. I’ve removed sheetrock and inspected the back side where different anchors have been used. It’s not a pretty sight. Having seen this first hand, again, I do not recommend you use them to hang anything of considerable weight such as the QS8’s.
As suggested already, I would run down to your local hardware store, buy a stick of ¾ X 3” hardwood (Oak) long enough to span between two joists and screw this to the joists. Then just mount the hangers onto the Oak.
