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!
Convert?fit=crop&h=128&rotate=exif&w=128kjweisner
why ? Was my first thought..as- a former 5a owner and now 7's...i never wished for another sub..
How well setup are the 5 ? I assume you have Vandertones ? And the RS analog meter and picked locations using the excellent manual?
Have you over equalized ? Trying to hard for perfectly flat will disappoint.
Post up some of your post EQ db #...use those 11 bands well...
Have you mucked about a bit w level and Q ? Those are a bit more subjective.
I think the third sub will just mess up the tuning..but hey, what do i know.
RV answers the phone...i would love to be a fly on the wall for that...
"why?" is a fair question! I’m only interested in trying it since I have the sub here just sitting idle.

Unfortunately, my current listening room isn’t really that great and I know I’m not able to get the kind of performance with my current placement that I’ve had in better room situations.

As for the tuning, I have done all that using the RS meter and the Stereophile warble tones (guess I could download the Vandertones and compare). Not trying for perfectly flat as I know that’s a fools errand--but thought "why not" since I have the sub. My goal isn’t to get more bass--rather to see if there’s a spot where the extra sub simply smooths everything out a bit and I don’t need to have a couple of my EQ setting either full on or fully attenuated.  

I have spoken with RV (different topic) and he was really helpful, but I know he can be a bit on the unvarnished side.  At any rate, I'm going to mess around just for fun and use a spectrum analyzer/pink noise generator as well as the Vandersteen method and see what happens.
Important, very to use vandertones, they are on the EQ freq centers and i believe ( not confirmed ) adjust for the RS meter.in essence calibrated.
I would reset using that first then add the sub...as you say, you have it !
I love unvarnished....
I talk to him about one time every moon, i will ask.
My guess after the grumble will be tune each side with it in circuit, measure at listening position..

My guess would be adding the sub would do more harm than good.  John Ruttan from Audio Connection in NJ would be able to advise you. Great guy who is very knowledgeable on anything Vandersteen and will go the extra mile (buying or not)  to help.

good luck,

Tom