The Focusrite is a good interface, but I wouldn't be surprised if it can't handle high levels. I have a 2i2, but I don't use it with my modular. I have output modules, which you no longer have space for (you might have to sacrifice one of the Dreadboxes for one if the problem persists). INST mode is off, right? I would be happier without the stereo adapter. I think you need to disconnect everything, and then see what combinations of connections and power on/off result in the buzzing. Is the interface connected to anything else besides the modular? Is it plugged into the same power strip as the modular, or a different power source?