I heard the Treos at RMAF and thought they sounded good but am hesitant to say anything else based on a brief audition in an unfamiliar room with unfamiliar electronics. I will say that, IMO, they might be the best looking Vandersteen yet, certainly far more attractive than the 2 or 3s. SQ-wise, I would trade up from my Sig IIs as they seemed a bit more resolved and transparent without any notable difference in bass extension.
Tonyjack, you should pose your questions on the Vandersteen website. Heres what I know (which may include wrong information): The drivers are the same as the Quatro except that the powered bass section is replaced by a single 8 driver (acoustic coupler to use Vandersteens word). So, the tweeter is a smidge better than the 3A. The mid-range driver is a bit improved over the 3A (tho current production 3As share this driver). Bass is handled by the same 6.5 as in the Quatro.
To me, the Treo is a different line of speaker rather than a spot between the 3A and Quatro. Sonically, they should be more similar than different. The Treo perhaps a bit more transparent and resolved but at the cost of lesser bass extension. The Treo certainly will look a lot better in your living room! But the 3A will continue to offer more sonic bang for the buck.