Vandersteen 5 / Quatro Owners--anyone adding a third sub?

I have a pair of Model 5s and also have a "spare" 2Wq that's been in storage (and is currently for sale here on the 'gon).  If it does not sell, I plan to play around with it in my audio room along with the 5s.  Has anyone done this, and if so, any pointers in terms of placement, etc?  I'm going to put it on wheels with a long signal and power cable so I can just roll it around, but would appreciate any points of view.

My listening environment is a long, narrowish rectangle--essentially a formal living room and dining room that are one big space, roughly the proportions of a shoebox.   Speakers are at one end against short wall and listening position is just a bit less than halfway down the long axis.  Left speaker in a "standard" corner and right speaker doesn't have a sidewall (open to entry foyer). 
Thanks in advance for your wisdom!