given your influences, how about a cheap casio or yamaha routed through the A4 for warmth?
roland paraphonic 505 is awesome and versatile (pricey and no midi) http://youtu.be/m6MdSc8H8_g
korg micro x (small, versatile)
but my fave is roland sh-32 i got iit just for my octatrack. check out this vid. it’s not me, but it shows what the sh32 can do: http://youtu.be/4GrYCHzEj58
fm is a good suggestion, but it’s not for everybody. and even after you detune the voices, you’ll need chorus, delay and reverb to bring out the real magic (yes, i know how to create chorus, delay and reverb via fm programming tricks, but outboard gear gives bette stereo width and depth and space.)