Positioning Vandersteens

I owned the 5A, and could not get used to the vertically-restricted sweet spot. I played extensively with tilt and toe-in, but the benefits were minimal. In my room the sweetspot was quite low, and  the speakers sounded best when I sat on a floor cushion. Otherwise, they just sounded dead.

Ok, now it makes more sense.  If you are hearing better low end you are probably experiencing a "valley" in the response at your normal position and probably a close to flat response in the upright position.  I would suggest getting a test CD with distinct frequencies on it then put low frequency tracks on repeat and move around your listening position and even the whole room.  You may be amazed at how much variation there will be in volume.  This is the heart of why so many people say the room is the most important component.  Sometimes you get lucky, other times it's a chore to get a room sounding good.  Positioning the speakers is the first step, but more may be necessary.  

I concur, Pmotz.
Knowing Richard, he will sort this out for the OP.
A test CD( or the Vandertones) and a cheap Radio Shack soundmeter should help locate the problem.