HI zeitwand on welcome on this board.
on the DrumBrute (not the Drumbrute impact), you'll see that in rear panel, in headphones section, you have a 3.5mm socket, a 6.35mm socket to plug your headphones, and a little button labeled "level"
if you push this button, it will make this button stand out the rear panel. From there, you can set the volume of headphones output by turning it.
You'll also notice that each instrument track has it's own level knob.
hope this helps,
cheers,
ps : this post was in the wrong category, i'll move it right away.