Yep. Your speakers are relatively easy to power but they have limits, the receiver has limits. The limits should be plenty loud but partly that depends on the distance you are from your speakers and the spl (sound pressure level, expressed in dB) you're trying to attain. Keep in mind an active subwoofer (one with its own amp, i.e. most outside of HTIB) doesn't need any power from your avr, it just gets signal but the subwoofer will take a load off the receiver from trying to produce those low frequencies that are the hardest work the amp does.
As was said, those two subwoofer outputs are basically like an internal y-splitter, just two outlets for your sub channel for zone 1. You can even split those with y-cables even further with no issue if you want multiple subs. Speaking of subs I'd go for the 12" if you can afford it...and look at the internet-direct sold subs (it doesn't need to "match" your speakers unless you like an aesthetic match). Altho the newer Klipsch SW-112 appears to be getting pretty good reports and is priced well if you shop around...
