actually the filters on the indigo and C are different...they released a new algorithm thats supposedly more "analog" sounding
regardless, the filters on the A are awesome...i had one for a few months..it was ill...if only i had known what to do with it when i had it..ahaha
there are other differences too...lots more memory to store presets on the b and c, more arpeggiator patterns, more oscillators on the B and C..
anyway you get the gist...