I have the AMC player and am very happy with it, but as jerikl stated, I haven't done any a/b with other CDPs.
I did a search for the NAD and came up with this, from Gramaphone Magazine, regarding the differences between the "i" and non "i" models.:
NAD upgrades popular CD playersMarch 4 2002**
NAD has launched 'i' versions of its C521 and C541 players, bringing enhanced performance and additional features to two of its best-selling products. The principal change is to the CD mechanisms used, which are now fully compatible with CD-R and CD-RW discs, and offer 'significantly reduced jitter' and improved tracking of discs, especially those 'burnt' at home on CD-R/RW blanks, and thus better sound than that offered by the original players. NAD C541i
The C541i has a flatter frequency response than the original C541, enhancing realism, but the players continue to use Burr-Brown sigma/delta conversion: 20-bit in the C521i and 24-bit in the more expensive C541i. Separate power regulation is used for the analogue and digital sections of the players.
The more expensive player, which offers HDCD compatibility, also has upgraded passive components, for example using Nichicon Muse capacitors in sound-critical areas for better performance, and a new remote handset is supplied adding more facilities. The player also has a standby mode, operable from the remote, an external infrared sensor or a 12v trigger connection, this ensuring better compatibility with multiroom control systems such as Crestron and Aegis.