You have to have the Audio I/O, yes. The reason there is because your incoming audio signals need to be boosted up from line-level to synth-level, and then back down again at the output. It's a decent interface for this sort of use, but you need to keep in mind that its inputs don't have envelope followers, so if you wanted to patch up something dynamics-dependant, you either have to add a couple of these or jettison the Audio I/O and go with separate inputs that have the envelope follower (the Doepfer A-119 is sort of the bog standard here) and then a stereo output with transformer isolation. That last bit is useful to keep noise and crud from passing from the synth to your mixer and vice-versa.
-- LugiaThanks for this advise Lugia! Hadn’t even considered that so very much appreciated. Is it possible to add a module that does this specifically after the Audio I / O?
-- TMR1984
Do you mean the output isolation or the envelope follower? If the former, there's no point...you'd be feeding an output module into yet another output module. No point to that whatsoever, as it wastes space and adds nothing.
Now, as for the envelope follower...yes, there ARE separate envelope follower modules, and you'll find them in their pulldown category. But the issue there gets back to "real estate"...would it make more sense to have a dedicated input module with the envelope follower in 8 hp, or to take another 4-8 hp for an extra module whose function would be covered by the former example? As long as you're in a smaller build scale like this, panel space needs to be treated like it was made of platinum.