It's been said that Klipsch centers don't perform very well and are known to have clarity issues. 2-way Centers, in general, are known to have uneven off-axis dispersion due to the layout of the drivers (read about Lobing).
In terms of sticking within the Big-K ecosystem, I can't really give you a recommendation. (Which is to say, my rec would be contrary to what you may want to hear. *shrugs)
Protect yourself and make certain you can get one to try in-home with a return policy that will cost nothing or have minimal stocking/shipping charges associated (like Crutchfield).
Otherwise, return the 600s and start over with a different brand.
![Wink ;) ;)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
JBL Studio 5 series are very popular and can be had on sale for a great price. One friend here replaced his RP 8000s With Studio 590s: though he said he missed the impact of the Klipsch, he also admitted the 590s were much better speakers with more accurate SQ.
Just my 2¢.