The phono stages in the C500 & 1000 are identical to each other, whereas the C2300 phono stage resides on the motherboard & in the case of the MC stage, does not have an independant MC module.
Having owned both the C2300 & C500 combo, there is a significant difference between the fully balanced units and the C2300.
More info here:
http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1280685205&read&keyw&zzc500
Bob