I'm a SQ guy. always SQ come first... functions don't really care much ....
if you are going to use amp + capacitors LOD and lossless files like FLAC with rockboxed + imod will be the winner
No amp - Rockboxed 5.5G Ipod has warmer sound and better SQ than original 5.5G.
while new classic 120GB (unable to Rockbox) is more transparent, less warm sound.
it is still depend on your preference