Is it easier to take a QS8 down and connect it to a front main wire? This way you will now for sure if it is the speaker. If the Q still sounds fuzzy at that point the speaker is bad and you can try to tell which drivr is gone, if the Q sounds good then the wiring to the surrounds could be loose or the amp section for the surround is causing an issue and the Onkyo would nedd to go in for repair
To tell which driver you should be able to tell with your ear listening closely to each driver. If they all sound bad it could be a crossover issue as well.
