For 600 bucks you really can't go wrong with an Aerial center speaker. For surrounds, well, not sure how much you're gonna notice an upgrade there. Maybe some Elacs?
https://www.usaudiomart.com/details/649493190-aerial-acoustics-cc3-center-speaker/
https://www.usaudiomart.com/details/649493190-aerial-acoustics-cc3-center-speaker/