Naim themselves state....
".. improved ceramic heat sinking, restructured cabling, and circuit
changes.."
...and of course - as you mentioned - the MP3 player connection on the front panel.
From another source...
"... the new 5i features ceramic heat sink material, which delivers
better efficiency and far lower capacitive coupling of the transistors
to the chassis. Restructured wiring improves the isolation of the
PCBs, the overall feedback has been reduced to lower transient
intermodulation distortion (improving stability into difficult loads)
and, finally, the passive preamplifier stage now benefits from improved
RF filtering."