It is pretty normal for any high-powered unbalanced (RCA input)amplifier to hum if nothing is connected to its inputs.
To be certain, short one end of a pair of RCA cables with paper clips, telephone wire, a piece of solder, whatever is handy and conductive, insert the other end of the IC on the amp's input connectors and turn the amp on.
By short I mean connect the center plug to the outer collar of the RCA plug. Better still, buy a pair of inexpensive RCA plugs from RS and sloder pluas and minus together.
This will NOT damage any RCA input amp or preamp.
I bet the hum will disappear.
BTW I used to own a MKII, great amp.
To be certain, short one end of a pair of RCA cables with paper clips, telephone wire, a piece of solder, whatever is handy and conductive, insert the other end of the IC on the amp's input connectors and turn the amp on.
By short I mean connect the center plug to the outer collar of the RCA plug. Better still, buy a pair of inexpensive RCA plugs from RS and sloder pluas and minus together.
This will NOT damage any RCA input amp or preamp.
I bet the hum will disappear.
BTW I used to own a MKII, great amp.