Absolutely!! I was running three identical Axiom bookshelf speakers across the front in my old system, and while using pink noise to set the levels, they all sounded decidedly different from each other. I even swapped the little devils around from location to location, but the relative sound differences pretty well stayed in place!
Another point on centers: If your seating in your listening room is fairly wide spread, a horizontal center like the VP100 will give you a somewhat wider sweet spot than the more restricted off-axis responce of a vertical bookshelf.(VP150, better yet!)
