Before you wait too long and somebody else takes it...use this older one as your benchmark...
http://www.ebay.com/itm/HSU-Research-VTF-2-MK-2-Subwoofer-Speaker-/360487808449?pt=Speakers_Subwoofers&hash=item53eebf6dc1
Maybe before you plunk down some cash on a new one...
Browse your local CL and Ebay(set it for "distance") and see what is out there...
My area...
http://chicago.craigslist.org/wcl/ele/3256321749.html
http://chicago.craigslist.org/nwc/ele/3236999939.html (boy do I wish...I haven't spent that...
First thing about subwoofers...
In theory, it goes where the "least null" point is. In reality it goes where you can put it that it has its best chance to do the job and not need "copious" amounts of auto-correct.
Dynamat on the sub for its feet would probably help. That might do the trick...
I'm trying to figure out why it is "off center"...
That room doesn't exactly promote the ability to add two more surrounds. Although two more exact, on the exact stand wouldn't be bad either. You'd have to move the surround to the sides, then the rear surrounds would go directly behind the couch.