Controlling the speed of the sequencer is the key to many unique sounds. It can't be done by assigning the sequencer rate knob to the modulation row. At least not with this/my firmware revision.
So you'll have to you the work around of controlling songtempo from your Daw. In the soundcloud example below I 've set midi sync on the MatrixBrute to external
In Ableton (are there other Daws
?) you record your sequence in arrangement view and scroll all the way down to the master track. Open the parameter view of the master track by clicking on the triangle and select song tempo as the parameter you want to control.
There are now two ways to control the sequencer speed: by clicking on the red line you create amplitude points that you can interconnect/move up and down to vary the tempo from 60 to 200 beats per second.
If you press "b' you can draw horizontal lines in the grid, thus linking the tempo to the rithm/beats of your track. Using CMD-1, CMD-2 and CMD-3 (on the MAC) you can change te resolution of your changes. This is only a first experiment. I think you could spend weeks exploring these options
See:
https://soundcloud.com/khidr/sequencer-speed-experiment