If the RX-V663 has a Power toggle, it is probably NEC1 device 126, function 42.  Use setup code 30376.  Test to see if Power toggles by:
1) Press Setup briefly (don't hold it)
2) Enter 36837 while pointing the remote at the receiver.  When the 5th digit is entered, the IR signal should be sent to the receiver.

If that works, use the 994 Command to assign the EFC to a button.  The Power button will probably be OK, but some remotes don't allow reassigning the Power button.