[jdavia]

I have other thoughts. First is The brand is top knotch but the mixer amp is 8 channels and really meant for a band. with more controls than is necessary for a karaoke show. I would think it is being used with some of the controls that are not in use turned up, or at the wrong settings, causing the hum. Turn off all channels and other settings that are not in use.

As far as the volume is concerned, I'd say definitely the speaker wire is too small in size and can be shorter if possible. The amp has 4 ohm speaker connections, make sure the speakers are 4 ohm, not 8, and only one speaker to a connection to keep the impedance matched. Also the amp is Stereo, so make sure both channels are turned up and in use.
I say this because I have a mixer amp myself by Martin Roland for Karaoke and DJ. When I first purchased it I had to learn what some of the settings had to be set to. Since I had a simular situation.