Alchemy, for sure, since it's the most flexible. impOSCar additive synthesis is too limited, really. IL plugins are 32-bit only IIRC (bad in my book). Razor is amazing, in fact, probably the easiest to make sounds with, but not in a purely additive way. Alchemy is for sure the closest thing to the venerable Kawai K5000 (and then some!).
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Have to agree with Alchemy. You have up to 600 oscillators per source, with 4 sources for a total of 2400 oscillators. In addition, if you turn down the Stretch knob to 0% and use the position know to move through your additive data, you can use each source as a wavetable with as many points of data as you want. And you can choose any number of waveforms as the basic wave for synthesis.
Another good option is Zebra, as that that lets you use quite a large number of partials as well, it also can be used as a wavetable, in addition to stacking/detuning the oscillators and using spectral effects.
For both synths, that's additive synthesis before using any of their other options (filters, fx, additional modules, etc). Plenty of power in both.
Yeap, Zebra is nice in additive works, especially when you can morph between 16 defferent "tables" of partials, and then use oscillator Fx...yammie.
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