Hello. This "dududu" sound, or motorboating as it's commonly referred to, is do to an imbalance in capacitance in the gain stages. It has to do with either the value of the cathode bypass cap, or decoupling cap of a particular gain stage. It often becomes apparent at higher volumes due to the increase in amplitude of the signal. I suspect, that Mesa pushed the limits in terms of having a higher value interstage decoupling cap, or cathode bypass cap, in order to increase bass response.
Try reducing the gain and bass controls when you hear this and tell me if it goes away.
