Chopper,
I installed four 6 1/2" Infinity Reference 6022si speakers, by enlarging the cut-out in the rear brackets as shown in Etlsport's pictures. The tweeters allow for a 3 db boost (selectable) for better imaging, and I like the mid-bass and bass performance. Also, these speakers are pretty efficient, and are a good choice if you're not amplified.

Randy