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I've never heard anyone talk of usb speakers. I got a set of Alesis active studio monitors, model 320usb, for my computer setup. They actually can accept 1/8", 1/4", RCA and USB. I got them on sale at musicians friend for $99. They have a very clean, accurate and entertaining sound at lower volumes. They will rock OK but not as well as a 3-piece system.
 
I have a pair of Bose Companion 5 speakers, and they are simply amazing. Most high-end desktop speakers have good bass, but the treble is not really good; this one has very good treble as well, though not as much bass as Logitech's flagship model.

It's $450 or something I believe though, and you might be able to find better speakers for the price; I got a nice deal on it so I only paid around $200, and its definitely worth up to $300 I think.
 
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