I had a major improvement with the AP2 +PP on my NAD M51.
However I probably had mine connected differently. NAD claim the AES/EBU is the best digital input so I used a 75ohm BNC to 110ohm XLR Impedance transformer. Direct connected the AP2 to the BNC on the impedance transformer. Sounded better than using that cheap little digital RCA connector, and it let the AP2 do its improvements.