I am not an audiophile. I'm quite new to utilizing quality speakers and none of what I am about to say is objective in the proper sense. This is all my impression based on how I enjoy sounds in my home. Some of the more informed audiophiles may be able to present you a rational/objective comparison and numbers to support their argument. I cannot as I've not compared the CB-20 speakers to any Polk speakers. Here endeth my disclosure
I bought a pair of Energy CB-20 speakers about a month ago. They're connected to my Onkyo TX-SR607 and are employed in general use (music, movies, gaming [XBox 360]). I made my selection by listening to the speakers extensively at a retailer, comparing them to similar Paradigm bookshelf speakers (I'm afraid I don't know what model). Having played the cello when I was much younger the clarity of this instrument in recordings biased my decision in favor of the Energy product. While the Paradigms sounded beautiful I picked out cellos just slightly better on the Energies.
I am exceptionally happy with the purchase. These are the clearest speakers I have ever attached to my entertainment center (that's not much of an endorsement, however, as my experience is decidedly limited).
I've put them through their paces with music (MP3s, AACs and CDs) quite a bit since I installed them. The sound is stunning and clear. Female vocalists sound, in my limited experience, beyond wonderful. Rock music is clear, well defined and fun to listen to. Classical music is so stunning on these speakers that I have begun to question how I ever survived with the Bose bookshelves I used for so many years.
I have also gamed heavily with these speakers, deliberately paying attention to the detail heard in:
* Forza 3 (the speakers make this game shine even more than it had previously)
* Battlestations: Pacific (the CB-20s large woofers make the game's big bore guns something to behold)
* Gears of War 2 (I can only describe this game's sound on the CB-20s with a BIG EVIL GRIN)
I stream between 5 and 10 movies per week on NetFlix via my XBox and they all sound wonderful. Blu Rays are also exceptional, Patton's soundtrack sounded particularly amazing.
Based on personal bias I would recommend the CB-20s or the similar Paradigm product. Objectively, I would highly recommend that you go to a retailer and compare as many similar speakers as you can and choose the ones which you enjoy the most.
To afrogt's point - my retailer did not have RC-10s in stock during my audition, last month. I was also given a nice discount on the CB-20s.