I am firmly in the full-range camp and use a REL Stadium III. I think a lot of people simply do not take the time to fully optimize the sub, which involves at least five major simultaneous variables: (1) room placement, (2) crossover point, (3) crossover slope, (4) sub output level, and (5) phasing. This is an extraordinary challenge that is beyond the desire of many (and the capability of some) to solve. I'm not trying to insult anyone, it's just that I think most people aren't willing to put out the effort to get it right. (Heck, even mine isn't fully optimized, due to home decorating issues...) But when you finally DO get it right, there is simply no comparison at all. And then you find that listening to a non-full-range system is just no longer satisfying. So like many things in high-end audio, it's both a blessing and a curse.:)
One other comment, which is not generally considered in these discussions: The combination of a high-quality monitor and an excellent sub, when set up properly, will almost always outperform a single large speaker system in a given room. (Caveat: any crossover point over 50Hz is too high to ever integrate seamlessly, so forget about tiny monitors that roll off at 80Hz.) This is due to the extraordinary flexibility of the sub/sat system in taming room modes in the bass while allowing for optimizing placement for midrange smoothness and imaging qualities. Yes, you can occasionally buy a full-range speaker and find a room placement which optimizes both imaging and the bass range (not to mention spousal aesthetic issues), but it is quite difficult, maybe even rare, and much more of a lottery than many people will admit. While the sub/sat system is not perfect either, it at least gives a much higher chance of actually achieving an overall optimum in the real world.
P.S. I would agree, I've never in my life heard an 18" cone that could keep pace musically. There's a reason RELs use 10" cones, even on their biggest subs.
