I have heard both speakers and after lengthy listening at different dealers, here is my opinion: IMO, Watt Puppy 6's have boomy and congested mid-bass and not much extension below. Otherwise, I don't find that they do much terribly wrong, but I think they are way overpriced at $20,000.00. The Vandersteens are the best speakers I have ever personally heard. They are not dark, as is the complaint with other Vandersteen loudspeakers, nor bright. They are smooth, natural, neutral, but yet, unlike other Vandersteen speakers, they make you take notice of the highs rather than the bottom end of the frequency spectrum. They deliver a breathtakingly exceptional midrange and high frequency presentation, with plenty of separation around instruments as well as a very wide and deep soundstage. The midrange transparence and immediacy is exceptional. More than any other speaker which I have heard, it felt like the musical performers were in the room with me. In the system in which I heard them, I found the bass a little bit soft and not as deep as I would have thought, but it was smooth. Since the bass is adjustable, perhaps this could have been altered to more closely suit my tastes. Dynamics as I heard them in that listening session were good, but not exceptional, as is my impression of other Vandersteen speakers, but this could have been a by-product of the setup which was Wadia 860, Ayre K1, Ayre V1 and the speaker cables were Nordost SPM.
The Watt Puppy setup was Proceed DVD transport (model unknown), Levinson 30.6 DAC, Hovland HP-100 preamp or Aesthetix Callisto linestage, and Levinson 336 power amp. Interconnects were XLO between DVD player and DAC and Siltech between pre and power amp. Speaker cables were Siltech.
The Vandersteen 5 is twice the speaker at 1/2 the price of the Watt Puppy 6's, IMO.