The bargain polysynths of the ‘80s were by necessity stripped of luxuries like cross-modulation, filter envelopes and even that precious second oscillator, replaced by enhancements like sub-oscillators and arpeggiators. The Polysix had these, along with some gooey chorus/phaser/ensemble effects to fatten things up.
The iPolysix is outfitted in much the same way, though a few additional treats are bestowed upon the modern user. For instance, the filter can be switched from 24dB low-pass to a high-pass mode. However, you're still stuck with a single four-stage envelope generator, shared between filter and amp and VCO - this last destination an addition in the virtual incarnation. The LFO (here called MG) is likewise sparse, with a single sine wave shape. The MG can be routed to VCO, VCA or VCF.
Such meagre modulation facilities may seem crippling in light of modern miracle machines, but in fact, the limitations of both the original and its progeny are part of the appeal of these instruments. Even a novice can dive right in, tweak a few knobs and achieve passable - even superior - sounds.
The iPolysix sounds utterly authentic, with that characteristic richness in the low-mids that is the hallmark of a vintage analogue synthesizer. The sub-oscillator adds just the right amount of grunt where it's needed, and goes some way towards making up for the lack of a second oscillator. The oscillator itself is agreeably ordained, with triangle, sawtooth, pulse and PWM waves along with noise. The basics, then, and done right.