The Onkyo 509 will drive the Sierra 1 or CMT 340 just fine. For same amount of power delivered by the receiver, the CMT 340 will be louder.
A lot of people do this so the idea is great! IMO, the CBM 170 make more sense as surrounds. Based on your upgrade path, I'd recommend you get only these for now. Many folk (myself included) run 2.0 or 2.1 setups for movies and the phantom center effect created by the L - R is sufficient. If you are eventually planning on the Sierra tower, the matching Sierra Horizon center is a requirement. That is the only way you will be assured a seamless frontstage (L - C - R).
Philharmonic Audio and Salk Sound make towers with sonic characteristics very similar to Sierra Towers. Make sure you check them out when shopping for those.
This will be a treat! I bet a Rythmik Sound subwoofer will be on the list by the time you leave and the Klipsch sub's days will be numbered
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