Fairlight-CMI

I think the Camel Audio Cameleon plugin is one of the best additive 
engines around at the moment. For hardware, the Kawai K5000 is the way 
to go. Either of these pretty much eats the IIx's lunch soundwise when 
it comes to additive synthesis.

If you just want to imitate (not improve on) the Fairlight's additive 
capabilities, though, I think the DX7 can come pretty close with a LOT 
more controllability.