If I were contemplating a sound system where the two main or front speakers had to be supplemented by a subwoofer, I would instinctively buy two of them and place the main speakers directly on top of them or at least as close as possible if they were say, panel type speakers. That is because I don't know how it is possible to overcome the inevitable phase interference problem in the crossover transistion region when the subwoofer and the bass speaker in the main speaker system are physically separated from each other by more than a few inches. You can see this on another currently runnig thread requesting advice on adjusting the phase control on a subwoofer. The answer as I see it is that even if you could get it right at one frquency, it would be wrong for all others including those near it.

I think it is outrageous for a manufacturer to sell an expensive "high fidelity" loudspeaker system and then tell you that if you want to hear deep bass, you have to buy another loudspeaker too. That should be part of the system to begin with IMO.

As for the spatial radiating pattern of a loudspeaker, the ideal solution is to have the indirect firing sound as controllable as possible to adjust for the different absorbancy of the rooms they are installed in. I didn't say it wouldn't be difficult to set speakers ideally or optimally especially when you have no control at all over their relative indirect firing output to begin with but ignoring the problem doesn't make it go away.