Mixing brands is a big no-no especially with in ceiling speakers thrown in. The most important feature in a 5.1 system is having all 5 speakers from the same brand and same series with the exact same tweeter for smooth all around voicing/timbre (this makes a huge improvement). If you must keep the Def Tech center then be sure to get Def Tech fronts with the same tweeter.

Have him sell the Def Tech center and match these with a good sub.

http://emotiva.com/shop/cart.php?m=product_detail&p=25

Read the review at Audioholics. This is a great system at an amazing special price.

RR6