The Quatro is a very neutral speaker with a great bottom end.
The 3.6 is a speaker a lot of folks like. I personally find it a little goosed in the midrange and adding a bit of sound that isn't really there. This deviation from neutral is what I think a lot of buyers find appealing.
However, if you sit 2 people side by side and listen to this speaker, each will get a different perspective because of the 2 panels. They only integrate in one area. This was the biggest drawback for me and I couldn't get past the back wall sound reflections in my room. I can see why a lot of folks like the speaker though.
The Vandersteen is much flatter in response and a little more dynamic. However, it does sound a hair dark *especially compared to the lively 3.6* and the soundstage perspective is back more from the front plain.
Let me sum it up this way, the Quatro is more of a tool for wanting a neutral and accurate sonic presentation (not always good) where the 3.6 is lively and appealing and with its little extra mids gives the illusion of more detail.
If you look at a frequency plot of the 2 speakers, this will be brought to light. Plus, the 3.6 does use a crossover for the 2 panels.