Haven't tried this with a "dumb" MIDI controller - since I do not own one. I have a Korg with micro keys - but it only has 5-pin DIN connectivity.

I did try Montage-to-Montage which doesn't work.

Hello Reinhard - I'm trying to figure out why you'd want to do that?

@Jason: Thanks for your reply!
@Rod: I'd like to attach a small external keyboard or pad controller to play Sounds from the Montage without adding too much complexity to my rig. Having a separate keyboard makes some things easier/more logical on stage
The Midi-In aren't available to me, as the Montage is already connected to Ableton via USB. I already control the Montage via Ableton, so routing a keyboard through Ableton would be possible, but I'm trying to make it as "standalone" as possible. Even thinking about replacing Ableton with the MPC Live

Although you could go the MIDI/DIN route with the external keyboard - Ableton receives this data through port 3 - then "echos" it back to Montage.

Using USB MIDI does not completely cut off the external MIDI in.

I use a midi-merger for additional midi-keyboards and a midi-foot-switch (Behringer FCB1010).
It is small device with four in, one out to the Montage. Works perfectly and has priority intelligence. I do not notice any delay.
With the external keyboards I address parts 9-16. It also has some pads with which i can trigger one-shot effects at one of the 9-16 parts of Montage.
The foot-switch I mainly do use for changing scenes, when I can't release my hands.
In Holland it costs around 50 euro.