This isn't a direct comparison but my experience has some overlap with what you are considering.
I ran a pair of 5140s off a Linn Majik (66W/channel into 4 ohms) for some time. Found a used deal on a 5103 then bought a new 5125 and changed from Majik to 5103/5125 in one move. I also added-in a 5120 center channel so the 5125 powered all 3 front speakers passively.
Total improvement was enormous and it seemed like I was running a different pair of speakers so if you can figure out how your NAD compares to a Linn Majik this might be a helpful comparison to the improvement you might see from moving to a 5103/5125.
I also have a 5-disk changer (for similar reasons to you) and use a Parasound CDC-1200 5-disk changer which has a digital output post in addidtional to the taypical analog outputs. After upgrading from the Majik to 5103/5125 and as Thsalmon suggested to you above, I bought a digital coaxial cable and connected the digital output of my CD changer to the 5103 which also had further, noticeable improvement in music detailing.
Since you asked above, I will explain this. Your CD player has a digital-to-analog converter (DAC) inside. This takes the digital signal read from the CD and coverts it into an analog signal which your speakers need to play. The signal then goes to your NAD which amplifies it and shoots it to the speakers.
The improvement I noticed (and Thsalmon mentioned above) is if your CD player offers a digital output post, you can send the unprocessed digital signal to the Linn 5103, whose own DAC will do a much better job converting the digital signal to analog output than your CD player, and which will improve the performance of your system.