I tried connecting my vintage Korg EXP-2 to the Expr. Pedal 1 input of the MatrixBrute and -- in the MOD matrix -- mapping Exp 1 to VCA with a matrix value of +99. Unfortunately, the entire 0-100% volume range was within the first few degrees of pedal sweep. Strangely, I got greater sensitivity by running Arturia MIDI Control Centre and setting Pedal 1 polarity to Inverted, but the pedal worked the wrong way round, of course.
After reading lots of expression pedal reviews and taking a look at the Roland EV-5 recommended by Sören Sören above, I finally opted for the Boss EV-30. Why? Because the chassis and the hinged plate are made of diecast aluminium. It's built like a tank -- far more durable that the EV-5 and much cheaper and lighter plastic alternatives such as the M-Audio EX-P. It has TWO Exp outputs, a polarity switch (not necessary with the MatrixBrute), and independent pedal-range adjustment for both outputs (by tweaking the MIN knobs). Because of the dual outputs, you can connect it to the MatrixBrute's Expr. Pedal 1 and Expr. Pedal 2 inputs with two ¼-inch jack TRS (stereo) cables. And then you can control two Matrix parameters simultaneously, either in the same (+|+) or the opposite (+|-) sense. For example, with an ultrasaw lead, as you depress the pedal the volume increases while the Master Cutoff decreases. This feature gives you a wide range of creative foot-controlled modulation possibilities. The Roland EV-5 is currently selling for €66 and the Boss EV-30 for €99. To me, the extra €33 is a worthwhile investment, considering that you get an all-metal chassis and plate, as well as the dual Exp outputs and the range adjustment. The superb Boss EV-30 adds an extra, infinitely variable expressive dimension to your playing. And you could probably drive your car over it or have your [ahem] gravitationally challenged lover park her/his massive posterior on it without inflicting serious damage (to the pedal, I mean). It's extremely robust! Highly recommended for all MatrixBruters.