According to the manual, the Denon's pre-amp out is 1.2V. And according to the Audioholics review you posted, the MPS-1 should be good to at least 2V. So I now doubt that's the problem. Your speaker impedance does not affect the Denon at all since it's buffered by the MPS-1. You may want to check your speaker to see if it is damaged. If your Ohm meter reads less than 2 Ohms, that's probably your problem. If you don't have a meter, connect the speakers directly to the Denon.