Random jumps in sound from Opus Quad

Hi all, can anyone help please?

I have an Opus Quad and am getting a random jump in sound mainly from the bass side of things. I've tried new cables, plugging the speakers in via a mixing desk and through the xlr ports at the back, tried through the zone vs the master, tried different speakers too, so it has to be the Opus.

Any ideas please as its driving me mad!