It is my understanding that the QS-4'a have a hybrid neodynium & titanium 1" tweeter- different than most of the other Axiom speaker models. Apparently the hybrid tweeters have a 1/2" voice coil compared to the 1" voice coil in the other Axiom speakers, such as the VP150.

See the post under Technical Questions, titled "Tweeter Differences" for more information.