Well, I like two sub setups better than one and find that monitors that have good bass responce to begin with work best. The Vmps 626 ribbon monitors mated with a pair of Vmps subs at my local dealer are devastating on full range music.
I've been using sub/sat systems ever since I discovered the REL subs. Prior to this, I could always tell the sub was "there", and so couldn't live with them. But with the arrival of the RELs, and a select few others now, subs have gotten so good that IMO the sub/sat approach is superior to full range. It's not that the fundamental sound quality is higher than comparable full-range systems, but that the added flexibility in placement and bass tuning tends to make the sub/sat approach superior in most real-world rooms IF you are willing to spend the time to set it up right.
Sogood51 makes good points. With my experience and observations, I would respond with the following:
1. Most subwoofers are active with their own built-in amplifier. And subwoofer mfg'ers are typically never noted for their outstanding amplification that absolutely control the subwoofwoofer driver. This fact alone dooms some to many subwoofers to a me-too type of bass. Not to say that this is a bad sound but it can certainly be improved upon. Therefore, a passive subwoofer connected to the best amplifier that absolutely controls the subwoofer driver should work better. Problem is that there aren't many passive subs out there. Neither are there many amps that offer this type of control.
2. In my limited experience, I believe that the vast majority of ic's and sc's (speaker cables) invoke a time-smear into the bass that once again provides a me-too type of bass that is ill-defined with an induced boominess as a direct result of that time-smear. So one should consider cabling as an essential here.
3. I would venture a guess that most 2-channel or 2.1 channel users of sub/monitor configs are probably more concerned with accurate bass and mid-bass reproduction (rather than just an accurate subterrain type of bass reproduction). The advantages of 2 good subwoofers should outweigh one good subwoofer(per Sogood51's post). At the very least, a one sub only config is performing double-duty for all bass reproduction and that's asking a lot for any single driver.
4. Given recent improvements in configuration possibilities, dialing in an adequate crossover should not be too much of an issue. Depending on the products of course. But the obviously best scenario here would be full-range speakers that can go very low where the crossover concern has already been addressed by the designer. For example, the Von Schwikert VR4 Series III SE's go down to 16 Hz. A speaker like this resolves several issues including the potential crossover dilemma.
5. The quality of the subwoofer (and especially the associated sub amp) makes a whole lot more difference than the size of the subwoofer driver itself.
IMO, I would think going the very best 2-channel amplifier/Von Schwikert VR4 Series III SE type of full-range speaker (i.e. 16Hz repro) route stands a much greater chance at providing an absolutely controlling bass while also yielding the least compromises. Assuming, of course, the cables selected induce a minimal or no time-smear.
As usual though, the only way to find what ultimately works best is getting products into your system to evaluate.