You shouldn't mix your front sound stage... In other words you want the fr fl and c all from the same series of speakers {not just the same company}, they are built and tuned to a certain timbre so they all match and don't cause havok in your front end...
I have heard the series 20's and I am not sure you are going to do much better for what you want to spend, plus not changing the center isn't going to give you much of a change at all since that is the bulk of your front, in most movies and formats the left and right turn into effect speakers, granted they get a lot of action but your center is giving you the bulk of your material...
If you just want a change, change the entire front end, let us know what you don't like about the existing setup and we can make some recommendations on which way to go...
I just heard a sierra 2 front stage and they sounded awesome if you could double your budget... you could always sell the s20v3 fronts, they still fetch $550 a pair and the centers are getting hard to find...
