I've owned a Korg Polysix for some many years, and I've often had the lid open for one reason or another, not least curiousity. This page includes some details of two major parts of the voice circuitry, redrawn for clarity. The full service manual with schematics for the Polysix is available online at Old Crow's Synth Shop, which has excellent Polysix pages.
Korg PolySix Oscillator
The Korg Polysix oscillator is interesting as it is a linear (V/Hz) oscillator, rather than the more common exponential (V/Oct) type. When the Poly6 was reinvented as the Poly61 with MIDI and digital parameter access instead of knobs, it also lost its real VCOs for DCOs.
Korg PolySix SSM2044 Filter
Korg apparently used the SSM 2044 in many synths. I'd be interested to know how much the various designs vary, if at all. The Polysix is quite an interesting case, as it runs the chip on +/-5V. This probably entails some of the other component changes. The Poly6 only has a single oscillator, so it uses the second input to cancel any DC offset on the output.
Korg managed to squeeze the whole voice onto a small area of PCB. The six voices only occupy less than half a single board inside the synth.