U-He RePro build 4365 (Alpha!)[FREE]

It is an excessively CPU hungry anti-optimized monophonic synthesizer, in essence a stripped down version of the Sequential Circuits Pro-One we're currently modeling.

The research part is, we have implemented the same pretty extreme model of the CEM3320 Curtis filter chip in our vintage Pro-One using 5 different numerical methods, each costing a very different amount of CPU. We wish for you to spot the "most analogue" sounding method. By that we wish to see if it is worth spending a lot of CPU or if we could get away with something cheaper. Furthermore we wish to discuss the following questions:

what differences do you spot between the models?
when do these differences become audible, i.e. which settings promote these differences?