Passive subwoofer...huh???

Ok, I am a dufus. I just bought an RBH VDS-10A passive subwoofer on eBay. I have a decent system; Threshold Forte Model 3A amp (200 wpc into 8 ohms); KEF Reference 103/4 loudspeakers; Parasound PLD-1100 preamp & Arcam Alpha 8SE CD player. Sounds fantastic, with the KEFs just a tad weak in the bass department. Didn't want a powered sub (too much like home theater, I thought), so I saw the RBH passive sub with its nifty 10" aluminum cone driver and said "Eureka." Naturally I bought it. No thinking, just bought it. Then it occurred to me; can I power it directly from my amp, or do I need a dedicated subwoofer amp??? If I drive it directly from my amp, should I run right and left channels into it (it only has one set of 5-way binding posts on the rear), or should I just run one channel?? Have I totally wasted my money?? I'm assuming the sub has an internal "crossover" to limit high frequency input, but I don't know that either. Doh!!! Help guys. -David (should be "KefKing" & not "Klipschking")