My opinion on the multiple sub issue:

 Yes, worth it for smoothing out response in the main listening area.
 Trade-off: Outside of my main listening area, such as adjacent rooms and beyond, there is a very noticeable cancellation effect that is not present with only one sub.
 Stereo vs. multiple mono: I choose stereo for the difference (as it seems to me in my listening environment) in psycho-acoustic effect in playback of as-it-happened, “live” microphone-to-track recordings with little or no “mix-down.” For heavily mixed productions where items are electronically “placed” in one or more channels from a single recorded track, I doubt I could tell the difference in an A-B comparison test. With differences in both preference and listening environment from person to person, if you have access to a more than one sub, spend some time listening with differing setups and find what works best for you.

Regarding crossover selection and the issue of which speaker will handle what range of frequencies, I take into account several factors. Some of these heavily reflect my personal preferences:

Do I wish for the same loudspeaker in all 7 positions? Yes. Is the goal of high accuracy speakers with true ‘full range’ at all 7 positions realistic for me? No.

Also, my listening expectations, since moving to a system with a subwoofer about eight years ago, have come to include a reduction of low-end inter-modulation distortion. This IM distortion occurs when, for example, medium-level signals in the 300 Hz to 600 Hz range are literally riding on large-level signals in the 40 Hz to 80 Hz range because both ranges are being reproduced by the same moving surface. IM cannot be totally eliminated, but ‘sharing the load,’ by letting a subwoofer handle the lowest frequencies, helps a great deal as I hear it. For years and years in listening to four-way full range speakers with 16”, 8”, horn and tweeter components, I was accustomed to this type of inter-modulation being present. Without realizing it at first, when I went with a good subwoofer about eight years ago, I missed the this effect and had my subwoofer gain too high trying to make up for the missing low-end IM effect. After some careful tweaking and ‘learning’ the new sound, I now find a reduction in low-end IM effect to have a more ‘clean’ sound.

If I crossover at 80 Hz, then my subs are handling the lowest two-and-a-quarter octaves, and the woofer in my three-way loudspeakers are handling the next three-and-a-quarter octaves. This, in my mind, is a reasonably even sharing. If I crossover at a higher frequency for technical reasons, I am increasing the likelihood of localization. If I crossover at a lower frequency, the sharing of the low end becomes more lopsided and increases low-end IM effect. So for both technical and listening preference reasons, I choose to crossover at 80 Hz.

One last point before I cease being so verbose. My powered subs can cruise in bass-heavy passages, without ill effect, at power levels two or three times that of my three-way speakers. By moving more of the largest amplitude signals to the separately powered subs, the rest of the system can cruise more easily at even loud levels, and is capable of more overall if I really want to push it.

Thanks for listening, everyone.