There are two primary parts inside each of your CD players, a transport that picks up the digital signal and then a digital-to-analog converter.
When you use the digital output on either the NAD or the Rotel you're picking up the signal after it leaves the transport but before it gets to the digital-to-analog converter in those players and sending it instead to a digital-to-analog converter in your A-V receiver.
So, with your three pieces of gear you've got two choices of transports and three choices of digital-to-analog converters. If the two transports are of equal quality and none of the three digital-to-analog converters are noticeably better than the other two you're not going to hear a big difference.
The advantage of using the digital output is primarily if you're going to send the digital signal to a separate D/A converter that is audibly superior to the one that's inside the CD player.
When you use the digital output on either the NAD or the Rotel you're picking up the signal after it leaves the transport but before it gets to the digital-to-analog converter in those players and sending it instead to a digital-to-analog converter in your A-V receiver.
So, with your three pieces of gear you've got two choices of transports and three choices of digital-to-analog converters. If the two transports are of equal quality and none of the three digital-to-analog converters are noticeably better than the other two you're not going to hear a big difference.
The advantage of using the digital output is primarily if you're going to send the digital signal to a separate D/A converter that is audibly superior to the one that's inside the CD player.