Thank you all! What an enjoyable set of responses. I'm sorry I lost track of this thread and only rediscovered it now. I'll hope it's not now poor form to reply at this very late date.
To answer a question from one early commenter: My own interest had to do with the possibility of creating an array of relatively affordable, highly musical bookshelf-sized side-surround loudspeakers for a home theater, summing to 8 ohms. This notion was stimulated by a theory regarding home surround encountered elsewhere on the web: that an immersive illusion is best achieved at home (as it is in commercial theaters) not with a single bipolar or dipole side surround, but instead with two or three small, identical conventional speakers on each side wall. With such an approach, though, unless one is prepared to incorporate multiple power amps, one must be careful about the summed impedance of each such side-surround array.
Another possible application would be a pair of (or even four) bookshelf speakers driven by a single one-channel or two-channel subwoofer amp with a steep low-pass filter (and/or with the speakers' tweeters disconnected) -- so, functioning as petite passive bass modules. This would allow wall or shelf placement of one or two such bass modules up higher than the wall's height midpoint... which can magically smooth out a typical listening room's dominant bass mode c. 50hz-80hz. For that unusual smoothing application, the bookshelf 'modules' needn't reproduce the bottom couple octaves of bass, so small to mid-sized is fine. But again, the array's final impedance must be managed carefully.
I didn't mention these odd projects at the outset because they didn't seem likely to resemble others' range of interest in the question.
Well, thanks again for the fascinating range of excellent options!