I, too, own a pair of Treo's (non CT) with a pair of 2wq subs.
I love this pairing and am reluctant to change it.
Though, I can see the temptation of getting a pair of Sub 3.
As I have been waiting for an Ayre KXR to come to the used market, and not finding any, I, too, have been considering using that money on a pair of Sub 3's. The Vandy website explains that the Sub 3 has upgraded parts in the amp as well as the drivers, so I can assume bass output is 'tighter' and more accurate-especially with the equalizer.
My listening is more casual, and I can't place my subs in the best positions in my room, so I think I will pass for the time being.
If you want a good opinion, contact Johnny Rutan at audioconnection (his Agon ID, too). He can give you the best information and will never pressure or sell you. He knows Vandersteen as well as Mr. V. and I trust his opinion implicitly.
Bob