Subwoofers.... one 10" or two 8"?

@leotis -
I found that same procedure you recommend (setting sub's phase based on minimizing bass output at the crossover frequency while while main speakers are in reverse phase to sub).  It was actually in Sound & Vision and submitted by Robert Harley as I recall.  I didn't do the out-of-phase-on-the-mains part but did try maximizing dbs at the crossover frequency using a tone generator.  I will repeat the procedure to confirm but it seemed like a large change to my almost 0 degree initial phase setting was needed....going to 140 degrees! Both channels ended up at the same phase value and I was not trying to read the actual setting at that point...just going by ear with what gave the loudest sound.  Anyway, afterwards,  I was not very satisfied with overall impact on music.  It seemed like soundstage had collapsed and (lower midrange?) was weakened.  At this point, I'm inclined to agree with Erik that this method may have unintended consequences.  Based on what else I'd read about phase differing across a range of frequencies this is, perhaps, not surprising.  I'll probably repeat the experiment BUT I suspect setting phase is going to be trial and error based on the overall effect on the music and not just max db at the crossover frequency.

If you sum the signals from the sub and main you get +3 dB.(less after room interactions) If you cancel the signals with reverse polarity you get -40 dB. Much easier to hear! Method works best when the sub is close to the speakers. If sub farther away then differential distance from sub and main to different points in the room varies more, and result becomes more hit or miss. Then trial and error may be the best you can do. Maybe you are referring to Harley in Absolute Sound. http://www.theabsolutesound.com/articles/subwoofer-basics/?page=3

leotis - YES, RH of TAS. I might have muddled things together having read a couple of pieces about that  method including the one at the TAS link you provided.  I think RH contributed as well to the Sound + Vision article I read.  Did see his name...maybe in the comments section.  

The subs are located right next to the main speakers so that’s not a complicating factor. Some say not to go this way, others recommend it. The folks at SVS were okay with it as they say their subs do not have to be "boundary loaded".  They appear to be giving acceptable performance in this position and just need a bit of tweaking. I’m inclined to repeat the out of phase/phase setting experiment though as I mentioned results after the first attempt were not "ideal" in terms of overall impact on the music.

Hi Leotis, 
The links you posted above are interesting and look as though they'll be helpful to me (once I can get my DIY subs back online, anyway - modifying their cabinets and amp right now), Thank you! 

To anyone interested, I repeated the out of phase/phase setting experiment using the on-line tone generator posted above and a Radio Shack SPL meter positioned approx. ear level at the listening position.
I ran measurements from 200 to 10 HZ for the Silverlines alone with volume at 200 Hz set to 80db. Observed values generally less than 70db for 50 Hz and lower (a slight bump to 70 & 74 db for 40 & 35 Hz, respectively). Decibel levels at 80 Hz were virtually identical (i.e., 82/83 db) for 0, 20, 80, 100, 120 & 140 Hz. At 160 degrees, a slight reduction to 80 db was observed. A value of 77 db was measured at 180 degrees. I did not find a huge null created by running the subs out of phase to the main speakers. I wonder if this has to do with the great disparity in speaker sizes (one 12" SB1000 driver vs four 3.5" mid/woofer drivers in the Preludes) or if it is due to "reinforcing" room effects. I did listen to quite a bit of music afterwards with connections properly matched between subs and mains and with the phase now at 180 degrees. Did not notice any of the negative effects previously reported with phase at 140 degrees. Possibly the ("sub") bass was a little better at 180 than at 0 degrees but it was very subtle if not a placebo effect.