Dual vs single sub

Sorry Im sure this is all over the forums but I only found old articles. Situation. I have Two SVS 3000s that arent really doing it for me. Thinking of trading it in on the Big one and adding another in a year or so. Any thoughts on Big single vs 2 Medium?
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I hope I did not make any comments that broke rules that hurt anyone, that was not my intent. Oh maybe profanity?
I think my purpose is SPL but as a result better sound.  I don't know if anyone has ever heard Phil Lesh hit these certain bombs, when that happens I want to feel it. During regular listening I want it to sound better than good, but never overpowering.  Listen, I'm not in search of bass that somehow overpowers the music but if the walls shake when something goes down to 35hz, thats kind of cool to me, whatever, but saying that I need the fullness. It does not fill the room as it is right now. I know the power should be there. They are placed right next to the open baffles. I need to experiment more.  Anyway I do not want to really shake anything I just want good bass. My system is moving upstairs above the garage that will be a designated listening room. (in two months?) I want it to sound good where it is now though. I know there are fans of different sub companies, I went with SVS for 65 days or something. I think I might add one more and if I still have a girlfriend I can go from there. Thanks for all the help. Im in the process of reading millercarbons suggested links. 
My system
Parasound A21
Homemade open baffle audio nirvana 10" drivers (backups)
2 SVS 3000
ATA Phono Pre 
VPI with a good cartridge

These sound different and I switch them out sometimes
mytek DAC/pre 
and a
Lumnous audio passive pre

Tannoy Glenair 10" speakers in process of repair

I have (4) JL Audio F-113 subs with my 2 channel.  You can never have too much subs. The problem is that you don't want to have so much bass that it drowns the mids. That being stated, I have a DEQX unit that can dial in the subs to work well with the mains. 

BTW, when I use my Hometheater equipment there are 4 more subs added to the 2 channel set up.

You almost never get stereo bass unless your room is very dead and you crossover high. I have a single sub crossed at 80Hz that’s decently far from my speakers, and in my wooden-floor open floorplan with no treatment other than a rug, I almost never hear where it’s coming from.  
I would say dual Rythmik F15HP, dual bundle brings it to <$2400, they are very “musical” and are +/-3dB down to 10Hz! I mean, if you have money to blow there is also their F18 :).