It's very difficult to say without listening to both types. However, notice that the JBLs are basically a direct radiating design. The QSs on the other hand are a reflecting design with its side-firing dual tweeters and vertical firing woofers.
I had a pair of Sony bookshelves that I used as sides and, after getting my QS8s, I discovered that I liked diffuse rather than direct sound much better. There really is a very large difference in the surround experience between direct and diffuse sound. It appears more spacious, enveloping and natural.
Also, very recently a member compared QS8s and QS4s and found the 8s to be superior.
