Fowl, I have multiple subs but not used in the way you would think. I have an awkward shaped great room that had incredible null voids depending on where the sub was situated. I have a SVS PB12 NSD/2 and when in front, no bass in back and vice verse. So I bought a pair of Deftech BP7001s with the built in 10" subs and 1500 watt amp in each tower to help combat this problem.
Only this brought up other problems, when the towers were set to large and the subs in the towers got the full range signal they sounded very bloated and actually hindered the bass from my SVS as far as the sound went. So I chose to set them to small and cross over to the sub at 40hz and this tightened everything right up. I then decided to add a 10" sub to my center speaker so that it could keep up withe mains (also crossed at 40hz) and then added another sub mid bass sub to my rear speakers.
Now am I insane, yeah that's what my therapist and GF tell me but I digress. Anyway by doing this I noticed something interesting. First off I was getting extremely tight well controlled bass with having a total of four 10" subs being used as mid bass subs and by having the SVS handle everything below 40hz and not having to deal with any of the mid bass, My SVS actually handled much better (although not as noticable as when running mid bass also. I got overall great sounding bass from all over the room and with the SVS handling just the low end, it hit harder and shook my carcus much better with tight well controlled bass.
I can say to get great bass the best 3 things you can do.
1. Buy two subs to control null voids and get better overall bass from both sides of a room.
2. Buy a Gramma pad to isolate the sub from the floor and control bass vibartions which hinder the sound.
3. Placement, Placement, Placement
my last recommendation is to buy a seperate mid bass sub but that's just picking nits, but in my room, did an incredible job and with the three GTG's I have had with A/V members they all agree that adding mid bass subs did an incredible job. Now by you adding a second SVS your gonna get almost the same effect.
