John
Here is one of the Harman white papers by Todd Welti that deals with why multiple subwoofers are necessary in a HT application. It solves several problems such as getting more output and accordingly lowering distortion, smoothing room nodes and obtaining flat frequency response in many seats not just the single sweet seat where you like to sit. The more subs you add the less problems with room nodes across more seats and the better low bass detail and articulation.
A couple of years ago I was watching an episode of 24 with my two sons and I was enjoying the LFE in my sweet spot. One of my teenagers sitting 3 feet to my left started to complain of very loud bass drowning out the voices. My other son 3ft to my right thought son#1 was just causing trouble and said he hardly heard any bass. Multiple subs to the rescue.
http://www.audiovideointeriors.com/images/archivesart/0106toolemultsubs.pdfYour question about stereo subs are another matter and from the way you describe your processor I doubt you can get stereo subs with a dolby track since mono LFE and mono centre is being redirected to both subs. The only processor that I know that allows for stereo subs and a separated LFE only output is the Lexicon MC12.
Anytime I have experimented with stereo subs I have split the L main channel signal to one sub and the R to the other. I set the processor to no subwoofer to avoid sending the same mono redirected bass to each stereo sub. I then run a dedicated line directly from the dvd player to the sub. Overall its more trouble than its worth IMO since it creates tricky phase issues between the individual subs and speakers. In addition you need to also adjust the LFE loudness separately at the sub.
If you want stereo bass effects and your processor permits separate speaker bass management, a better way to go would be to cross full range speakers like the M80 at 40hz and run a pair of mono EP500s .
