Hi Daniel, every pre/pro has a low pass filter in it. You have a screen menu or a section in your manual that describe this. For me I prefer to use the variable crossover in the M&K sub since it has a better filter 36db/octave, usually the pre-pro has 12 or 18db/octave filter and a fix frequency at 80Hz, so that means you still have mid bass going to your sub. For my installation, my pre/pro is set to 12db/octave at 80Hz and I finalize my adjustment with my v-125 at around 70Hz, that way I use the 36db/octave and have no voice coming from my sub, only deep bass.
