Follow up
Thanks to those of you who took an interest and responded! I can't emphasize enough that, although I'm coming at this from my own biased starting place, I in no way consider my POV to be "right". I really appreciate the experience and opinions of others that probably have more HT experience than I. It's doubtful that I'll change my own plans since I'm already 1/3 to 1/2 way there ($$$-wise) and it's as much a matter of satisfying my own curiousity, since it seems to be such a rarity in the HT world. But hopefully we can all pique the interest of others (esp. the HT press!) in exploring the full range of options.
Something that occurred to me after my original posting that prompts a possible 5th question:
Almost all subwoofers are active. In fact, it seems to be almost assumed that they are. Few, if any, receivers that I'm aware of have an amp for driving a sub (I'm sure some do, just can't pull one off the top of my head). I don't remember any 5.1 or 7.1 stand-alone amps either. So the next question...
5) Why is it assumed to be desirable to design a dedicated x-over, amp, cabinet, and driver combination for a larger, low-frequency driver, while the lo-mids, mids, and highs are left to chance combinations the consumer may stumble upon?
Also, cornelius brings up an interesting point. As more hobbyist and professional musicians are bringing gear from Guitar Center into their homes, and Apple's GarageBand is enabling everyone who dreams of being a musician, it seems likely that the already-blurry line between pro audio and consumer audio will grow much dimmer in the very near future. The better for all of us!
Finally, mudcat, I believe that one can use combinations of amp and speaker to enhance particular traits of sound, and the fact that so many hit records of the '80s were mixed on NS-10s with heaven-only-knows-what-amp driving them certainly points to the power of the human brain in processing sound. If you "mixed based on what [you] would listen on", but you can't determine what your favourite store-bought movie or CD was mixed on, what would pick as the best compromise, especially knowing that there's a reasonable chance it was an active system? Being musicians and engineers, I'd like to know if your current listening system is active or passive, and what you think are the strong and weak points of it?