I absolutely want to be able to use an external sound source with my system. I’d quite like to compare the Nord Lead oscillators using the Nord Lead internal filter to the same oscillators using the modular filters for example. Or to use Thor as an oscillator with MIDI-control of the modular envelopes. Part of the point is figuring out what stuff actually sounds like. And part of the point is building grotesque Frankenstein patches.
There are various “external input” modules available (like the Doepfer A-119) but as far as I can understand I won’t need one. The modular should be able to accept a line-level input from my mixer directly.