Your Moab speakers (interesting speakers by the way) are 98dB efficient. As such and this is not always the case but....
Transistor power amp distortion characteristics are often dominated by harsh-sounding crossover notches at low power output settings. This effect disappears in class-A designs, which are well suited to driving high-efficiency speakers.
Transistors do have their "optimum range" of peak sound performance. Listening at a fraction of a watt is not something manufacturers of bigger solid state amps tune their amps for.
I would be very surprised if there is something wrong with your ML amp. But for the sake of peace of mind, I guess you should send it in if it is under warranty and all that.
Try a tube amp if you have that option on your speakers.
