You can run the monitors "full range", connect the sub from the pre-out of your preamp. If your preamp has only 1 output, you can use an adapter or cable splitter. -In my experience-, using a decent adapter does not degrade the signal. Then the trick is to keep the sub crossover and volume level low enough to not call attenuation to the sub (get the phase right). You should almost forget the sub is there, only missing it when it is gone.
Preamp, Amp, Subwoofer
I have a nice set of monitors. I have considered adding a subwoofer to extend my lower range. To do so properly would require that I run a set of interconnects from the preamp to the subwoofer and then from the subwoofer to the amplifier. My question is, won't all of this interconnecting and introducing a crossover into the circut between the preamp and amp negate all of the research I did before purchasing my preamp and amp? Which by the I am very happy with.