Thinking about this a little more...
I imagine that there were many people for whom the idea of a mono-synth was not very exciting, and it was only when you added Repro-5 that they paid up, and had access to both. Does your survey data account for those that might never have bought the original mono version, but then grew to love it due to it coming along with the poly version. Isn't it conceivable that many of those customers have now been persuaded by monophonic charm, and would be happy to buy more in a similar vein?