Your RTi8 towers aren't bad, it's the CSiA6 that most urgently needs replacement.
If you wanted to stay within the Polk family you could pick up the LSi 706C for $400 from the Polk Outlet online.
If you want the VISUAL uniformity of a "matching" front 3 then make a beeline to the Harman website and get a pair of the R263 towers on sale for $500/pr with the 3-way RC263 center for $350 and you'll be all set.
Infinity REFERENCE 263 | 6-1/2" 3-Way Floorstanding Speaker
Infinity's Reference R263 three way floorstanding home speaker offers detailed, spacious, room-filling sound that will bring your music and movies to life.

Infinity REFERENCE RC263 | 3-Way Center Channel Speaker
Infinity REFERENCE RC263 dual three way center channel speaker accommodates with any home theater system & has crystal clear sound quality with low distortion.

No need to spend more money unless you simply WANT to.