On paper, monitors + subs will almost certainly "outperform" equivalent price floorstanders if you only look at the raw numbers.
The problem is this: Most subs are designed for loudest deepest possible bass in smallest possible box, and such details as sound quality and integration with main speakers are often not given high priority. I have owned, sold & built monitors, subs, and floorstanders.
If you do go the monitors + sub route, I highly recommend you get two subs. Look for models that have a 4th order lowpass filter on the subwoofer, as rapidly rolling off the upper range of the subwoofer will avoid detriment to the upper bass and midrange region.
Many subwoofers have a high pass filter to roll off the bottom end of the main speakers. They inevitably use cheap capacitors in this generic filter. I do not recommend using it; in my experience the result is loss of liveliness and greater listening fatigue. If you want to protect the satellites from low frequencies, either use an active crossover or a passive line-level crossover between preamp and power amp (which will necessitate sending a line-level signal from the preamp to the subwoofer).
Although in general fullrange floorstanders are smoother in the bass region, imho a system using two or more high quality subwoofers can potentially deliver very smooth in-room bass if set up properly, and may even surpass the fullrange speakers in this respect. I can go into detail on how this is possible if you'd like, though I've posted about it in other threads.
Duke
