Perhaps you should try calling Vandersteen, and explain how you have this sub hooked up... Someone there should be able to figure out what the problem is.
Two possibilities - first, and most straightforward - your old sub may be more efficient than the new one, which means it will play louder. The other possibility is that the placement you're using is not ideal.
Try a different spot in the room. Just because the Velodyne worked in one spot doesn't mean another brand will work as well in the same spot.
One trick to find the perfect placement for your subwoofer is to put the sub in your listening position, and play music with a walking bass line. Now get down on your hands and knees and crawl around the room - into corners, along the walls, everywhere - until you find the spot where the bass sounds the best (or in your case, where the bass is the strongest). That's the spot where you should put the sub.
Your V2W may be operating out-of-phase with the main speakers. Try reversing the leads and see what happens. If that doesn't result in better bass response, then contact your local Vandersteen dealer (rather than call Vandersteen Audio -- Richard Vandersteen will probably direct you back to a dealer to resolve a problem like yours).
Is it possible that your system would prefer the external high pass filter being set one setting up or down from the actual input impedance of your amplfier? Sometimes one step change can make a huge difference.
Also, try the volume control in the opposite direction from your current setting. As you move up on the efficiency scale, I think the volume is lowered as you are telling it to mate with higher efficiency main speakers ... see what it sounds like as you tell it your main speakers are less and less efficient.