It's basically a 2.5 way version of the 2 way s30. It has the same drivers in a bigger body. The sound of CA Sirocco series is a little "dry" so amplifiers like nad or marantz will sound better on it than amps like yamaha or cambridge audio.
For 18k it's a fine speaker. Looks nice as well. Obviously not high end but if one wants a floorstander only within that budget then it's fine.
However I'm not sure how much a polk audio rti a3 bookshelf costs. I vaguely remember 23k (correct me if im wrong) if it is 23k the a3 will sound a lot better over all compared to the s70.