As I’m looking to upgrade to ModyPoly and/or Polysex, am I right to assume I should be on the lookout for a Polysix without any other mods already installed (if it already had the KiwiSix installed for example, would it make installing these more complicated/impossible)?
The Polysex mod is compatible with the Kiwisix
From the FAQ:
Q: Is the Polysex mod compatible with the Kiwisix retrofit?
A: Yes, but it uses the effect section LFO (0.1-7Hz) instead of the PWM LFO and the MG LFO is a square instead of a triangle waveform. There are detailed installation instructions in the construction manual.
ModyPoly+Kiwisix is possible but would require further modification. You will also loose features, so I do not recommend it.
Any other midi retrofit can be removed and replaced by the ModyPoly
On the KiwiTechnics’ site, alongside the KiwiSix description, they state “we no longer trust the original power regulating circuitry in the Polysix” and offer a power upgrade alongside the KiwiSix.
Is something like this really needed or would I be fine with just finding a nice unit with just the battery replaced and then installing the Tubbutec mod(s)?
While I do not want to contradict the experience of the Kiwisix guys, I personally never encountered a Polysix with a bad power supply, or a Polysix damaged from over voltage. (And so far I had about 50 different Polysix on my work bench).
So to answer your question: No, I don’t think something like this is really needed.