Mac's are not known for having great speakers. You really should consider a new center. USAM has multiple pairs of Def Tech speakers listed that will match the sound of yours. You don't even need a powered sub in the center so something like a 450 might work for not much money. Buy a pair and only use 1. Or, if you want to go crazy, there is a pair of 7004's listed...
Otherwise I totally agree with comments above about going "phantom" for the center.
Otherwise I totally agree with comments above about going "phantom" for the center.