I remember Harman making a bit deal out of multiple subwoofers with the HK990 (despite that, the thing has bugs in it's subwoofer routines - go figure :rolleyes
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. The idea being lower distortion and so on. From a somewhat different POV, multiple drivers reproducing the same signal (ignoring time alignment or comb filtering as problems) should mean less excursion and lower distortion. The multi-subs for more re-enforcement and "smoother" response is something I've seen published from THX. The resultant configurations in a given room for these two solutions will look very different.
Regarding "stereo" subwoofers - it depends on what you mean by stereo. Some older surround processors will actually compute the LF output in stereo (I own such a device), and hooking up two subwoofers to them is preferable to a single unit (although in my scenario, the two subs are stacked one on top of the other in the same place - I've tried them spread out, and it didn't do much for me, but it sure took up more floorspace!). This is different than modern devices that can handle multiple concurrent subwoofer outputs, but either separately level or (if they're really sophisticated) EQ them (like the Levinson 502, it only works on "one" LFE channel, it can just ("just" - it's one of the most sophisticated cinema processors ever made! :tongue_smile
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EQ four output subwoofers to take that one channel (the summed LF and the LFE)). I think the true "stereo" devices are relatively rare these days (in my processor, it actually takes all of the "left" channels and drops them on one subwoofer, and all of the "right" channels and drops them on the other subwoofer, and LFE is sent to both; the way the manual would have you set the thing up it's more like having two true full-range speakers as your L/R than having a subwoofer).
Personally I like the "stereo sub" setup on the processor that does it that way, but I would not hook up a similar arrangement on a processor that only has a single sub out (e.g. with splitters). If I was re-doing everything I would probably look for a more modern controller that could at least collectively EQ the subwoofers (the Velodyne controller can do this with two or three (I forget) subs, they aren't "independent" but the device measures the overall system with however-many subs it can hook up to, and then goes from there), as opposed to simply trying to have a ton of woofers throughout the room. I should also mention that I'm not a big subwoofer fan in general - I only have subs on the "stereo" system. Everything else I'm fine with letting it go over a cliff at 40hz or thereabouts.
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(I've just accepted that I don't like a lot of sub-bass).
Oh yeah, I agree with bigshot on placement - if you haven't done the bass crawl at least, you should do that before considering adding multiple subs. In my current setup, I just stacked them in the best place I found for one, because I didn't like giving up so much space to both cabinets. It isn't perfect, but it works well enough for me (or as they say, "close enough for gov't work!").