I would say 2 subs is best but go one at a time if money is an issue, I know 1 is not ideal.
Vandersteen 3A Sig listening window
The owners manual states that the "6 inch high optimum listening window is centered at 35 inches when the speaker is vertical". Now is that 35 inches off the ground or 35 inches off the bottom front of the speaker cabinet? The reason I ask is because the tip toes add an additional 1.5 inches to the height of the speakers when installed. What I'm looking at can I measure 13 inches off the top of the cabinet and use a laser pointer to mark the exact point of my listening postion?