Well they seem ok for the money, but have you considered the height factor? These stand 3' high and the speakers themselves are up top. Probably the upper 1' of the 3' height. I ask because your new center will be placed 'where' in relation to this height?
Meaning, if you buy these and then a center, how will placement play out? Will the center be around 2' off the floor? Alligning the center on a horizontal plane with the mains can really help the front soundstage IMO.
Just throwing some ideas out there.....
(I assume you've at least listened to them...yes?)