I regularly watch movies at -15dB in a much larger room that yours and have no issues. It's less about how loud a speaker can get, and more about how it handles dynamics and distortion at a high volume. Pretty much any speaker can handle having the volume cranked up to -15dB, but that doesn't mean its going to sound good. One thing that klipsch speakers excel at is high volumes with low distortion due to high efficiency, even among their smallest speakers. Purely from a power handling point of view, rb 10s are capable of reaching 95dB (-15dB in a properly calibrated system) at 3m in an anechoic chamber with a mere 35w, and that's taking into account Sound & Vision's more conservative sensitivity measurement of 89dB 1w/1m, so you should have no issue getting that loud, or louder.
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