a technical question

[K9 Kai Niggemann]

last saturday we played a gig in Nürnberg, (almost) everything went really well, was well received and I will describe the gig and the setup in more detail when I get the video too (I received some nice pictures yesterday).

But first I need to ask a question, something I don't really understand.

We were jamming with Max and Ableton Live on my MacBook Pro 17" with an RME Fireface400. 

input 3/4 got analog mic signals and the input from my Korg Monotron, whcih was playing a very low, kickdrum/heartbeat pulse, doubled with a delay. 

When I turned this up high to make it really painful and loud, the Fireface just shut down. No output at all, just complete silence. I turned down the MT and sound came back. I tried again, thinking I wasn't pushing it as high, but the FF shut off again. (imagine the look on my face...)

I set the Monotron to the side and everything went well.

What happened there? What is the physical cause of this behaviour? How do I avoid it (besides not using the analog synth in the FF400 inputs)?

Any help?

Thanks!
Kai