Well, let's try saying this.
Monitors don't hum.
Where is the hum coming from? 99% likelihood it is the speakers, thus being almost as likely some type of feedback. Check the power source for AC ripple, swap in a different JAMMA board and see if the hum goes away. Hum goes away, your problem is the MVS board. Hum stays, it's in the wiring or power supply.