First of all I want to applaud Arturia for the wonderful job they have done on recreating this keyboard, especially the enhancements over the original device, however, as a former owner of a real ARP2600 I noticed a few problems with the emulation:
First, the sample and hold output is a bit lagged between voltage changes, I.E. you can hear a certain amount of slide between the notes in the output. It should be a crisp change.
The Sawtooth output of the main VCOs appears to be reversed. I.E. The ramp voltage travels from high to low rather than low to high. This can be heard by linking a LFO output into VCF and listening to the output frequencies.
The Sawtooth and Pulse (square) waveforms in the standard VCFs and LFOs and clocks are supposed to be positive going only. The cycle is not supposed to fall below the zero voltage. Only the Sine and Triangle are supposed to do that. Peak to peak of any wave form is an absolute 5 volts. A sawtooth and square should run from 0 to 5 and a sine and triangle should run from -2.5 to +2.5
The Keyboard should zero voltage at the lowest key, not middle C and again run from 0 to 5 volts across the keyboard, if I remember the range correctly.... (its been about 25 years)
The filter should be able to be tuned/tempered to the keyboard on full KBD control voltage. I.E. You should be able to cause the filter to ocillate with the resonance, and with full KBD control voltage be able to play the filter as an ocillator, in tune.
I don't know if I'm missing something here, but it would be nice to be able to link the microphone input of the PC to the pre-amp input. A lot of cool patches can be made using the 2600 as an external signal modifier
These are the few things I found so far in the demo. I hope this helps to make the software a bit more true to the unit and easier to work with.
I'll keep poking around in the demo and see if I find anything else.