2 conductors vs 3

3rd conductor could be used to ground chassis but not required, could cause a ground loop, and I would be reluctant to do it without knowing the design.  Chassis grounding is required by various safety rules unless system meets certain safety requirements, often referred to as "double insulated".  apparently your rig is "double insulated", as are many other appliances with only 2 conductors.

The 3rd conductor is for safety rather than sound (generally). 

A heavier conductor (for the other 2) will likely help.  that is why we upgrade.

Having an unused 3rd wire will not hurt anything. 

Jerry

@miklet1

I wouldn’t hesitate for a second to replace that cheapo 2 conductor 16ga cable with a good, doesn’t have to be expensive, 3 wire after market power cord. I would probably look at, minimum, a 12ga conductor power cord. Try before you buy if possible... You will hear a difference for the better!

As for the ground wire in the cord. The IEC inlet connector on the back of the Marantz receiver does not have a ground blade. Two blade only.

There are 2 benefits to using a 3 conductor IEC cable instead

1. Better grip thanks to the ground pin in the IEC socket
2. Some grounded power cables are also shielded, but most are not.

Of course, you only get benefit #1 if your IEC socket has a ground. For this reason I upgraded my Luxman’s socket. The ground is not internally connected but it increases the grip strength by about a third.

I also use shielded power cables to reduce the chance of picking up noise from all the networking and streaming gear I have behind the rack.