@ihcho are you running the 4 or 8 ohm speaker taps on the back of the QS Monos --
- 8 or 4...with your 4ohm 90db Tyler speakers?
- 8 or 4...with your 6ohm 87db Gershman speakers?
- what preamp are you using to drive the QS monos?
A 3db speaker sensitivity difference there worth noting. Other things to check.
FYI, a member on another forum was having unsolved headaches driving his 87db Gershman Grands’ with his powerful Bryston 14B amp (600w@8ohm / 900@4ohms). At first everyone thought it was a preamp gain / output voltage issue not able to drive the amp(s). As it turned out, he had his amps and gear all plugged into the same AC wall plug / circuit. To fix it, Installed two dedicated 20amp circuits, problem solved, amp was able to drive the Gershams Grands ok. 1) Check, try the 4ohm taps on the Monos if you have not done so yet. And, 2) try plugging QS monos in to a different wall plug not on the same circuit as the rest of your gear. Best of luck.