Whatever little speaker you decide on, it will sound much better by, as others have already said, putting a high-pass filter on the amp powering them. That will remove the bass frequencies (depending on the corner frequency of the filter) from that amp and the monitor, greatly reducing the distortion the speaker produces. All the filter needs to be is (usually) a capacitor soldered onto the power amp's input jacks (on the inside of the amp). The formula for determining the value of the capacitor needed is:
C = 1 / 6.3 x f x z, where C = capacitance in uF, f = the corner frequency of the filter desired, and z = your power amp's input resistance figure.
This will NOT be possible if you hook up your sub to the same power amp as the speakers, obviously.