Basically anything you get is going to trump the Klipsch with great ease. I had a Klipsch Synergy 10" subwoofer. It sounded alright, but not great. I have also had an HSU STF-2, while I did not do a side by side comparison I noted the HSU went deeper, when louder, and was way cleaner sounding than the Klipsch. The HSU was also higher build quality.
SVS and HSU are probably the best known of the Internet direct subwoofer manufacturers. Another good ID company to look at for subwoofers would be AV123 and Axiom, each company has a wide variety of subwoofers to choose from.
And for the Big Box stores, Velodyne is the supremo of subwoofers. They have one of the largest variety of subwoofers of any manufacturer with a huge range. They have little $200 subs, multi-thousand dollar subs, and everything in between.
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