I tested tonight and the monotribe has plenty of gain on its own to go into the minibrute's audio input without an extra gain stage. I have it running thru a cheap mixer first, behringer 1002fx, but it is not necessary ( I am mixing multiple sources into the minibrute audio input, typically) though I do have to position the MB Audio in fader slightly higher without the mixer, I do not see this as a problem. There is no need for any "special" equipment to connect the monotribe other than the slightly odd TRRS cable (do yourself a favor and either make your own or get the nokia av cable from ebay) .
Do a search and it is mentioned on this forum that a resistor or 2 can be changed to increase the sensitivity of the audio input, but its surface mount and will void you warranty.
An attenuator is only necessary for the pitch CV on the Mg Ring modulator oscillator (ironically, its audio level is more than adequate) because it does not adhere to the same cv standard as the mini brute, really an easy fix. The monotribe will follow pitch if you follow korg's calibration procedure (and can be detuned in the same procedure), and fresh batteries are also key.