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.