what tweeter did you use for the center channel, there are a couple with faceplates that might just drop in that have very high sensitivity and should give a better result.I used an older version of this one from MCM with my nsb arrays that were very similar to yours 92db/1w, not bad considering the price
http://www.mcminone.com/product.asp?product_id=53-572&catalog_name=MCMProducts
Or a much better tweeter is this one 96db/1wm, and still a bargain at only $30
http://www.gr-research.com/index.asp?PageAction=VIEWPROD&ProdID=109