How to connect a diy audio signal to my "expensive" audio interface
Don't use the mic inputs, they're not for that.  You plug microphones into microphone inputs. The phantom power level of 48V might fry your Due as it's a DC level designed to operate active microphones.  Kit is interconnected via line I/O.  That's a higher level signal but line in &amp; line out are specifically designed for this.  So use your line input which must be available on a €4k DSP.

This will tell you all you need to know:-



The final thing to know is that the input impedance for a line input is circa 10kΩ, so pretty high.  If you drive your line out with a simple op amp follower, you'll just impedance bridge between the two and all will be well.  Only you can decide if your DSP is consumer or professional level.  If you start with consumer based line levels, you'll be fine and can then see if it feels as if the input can take a higher rms level.

Since you've built your kit yourself, it's likely that impedance bridging (low -> high) will also allow use of very high impedance inputs. These will be labelled high-Z and might be of the order of 500kΩ. They're for piezo equipment or a guitar but should also be okay for your Due.

A tip is to feed your kit's output into a PC audio card's line input as a level test.  If your PC doesn't toast and you can hear it, then there you go...