In selecting a monitor you'll need to first determine which is appropriate for your studio. If you have a small room you may benefit from a smaller subwoofer, because once the music is played the bass will attempt to disperse. If the subwoofer is too powerful you won't be able to really get it to a point where the amp is driving the speaker. In this case the bass will overwhelm the room and it will become detrimental to your cause.
Instead, try a smaller speaker with a less powerful amp, which should work well. You'll be able to crank it up a bit more than with a larger amp.
The subwoofer crossover should be set where the frequency response of the main speakers is at -3 dB.
Set it too high, and the subwoofer will play more midrange sounds.
Set it too low, and the main speakers and their corresponding amplifiers will run out of headroom faster and result in a different frequency response.
Room modes are common in small mixing studios. This can cause peaks and dips in frequency response in the bottom two octaves: - 20-40 Hz and 40- 80 Hz. You can often position the subwoofer to deliver flat requency response, but you can't often move main speakers.
Measure each speaker from the mixing position to establish how low it plays and set that channel's high pass filter at the -3 dB point. Set the subwoofer's low pass filter at the same point.
Pink noise will build up in the room, giving a false reading. A lot of listening to confirm the crossover point will help. Tenor voices help in gauging the crossover point.
If you don't have variable crossovers, choose something not higher than 100 Hz and avoid lowering the actual low frequency response of the main speakers. Pay close attention to subwoofer level calibration and phase.