Sub37 - external in/feedback

[macsaif]

Hello,

I am planning to add 2 additional VCO´s (diy Model D clones) to SUB37 via external audio in. Is there any possibility to switch between internal feedback and Audio-In or it is done directly in the input jack?
Other possibility is that I mix the audio out of SUB 37 to the signal of 2 VCO´s and connect it to the Audio In. Is that solution comparable with the internal feedback (sonically)?

The third solution is that I place the VCO board inside of the Sub37 and hardwire it. I can solve the control via MidiIn.

The warranty is not an issue, it has more than 2 years.

[DrJustice]

The external input jack is normalized so that when nothing is plugged in, you get the internal feedback. If you plug something in the feedback is disconnected.