Thinking outside the box, and possibly at the upper end of your range, but have your considered the aptly named Squeezebox Boom
Squeezebox Boom FAQ - Fab4 SqueezeboxWiki
There is a line in, that you can connect to, to use them as amplified speakers.
If you have access to wi-fi, or are allowed to connect devices to your network, that connects to the internet, then you can connect to internet radio, pandora, etc without using your PC. If this is not the case, then this won't be such a good solution.
Sound Quality is impressive for such a small all-in-one package. I've just tried it with some lossless files out of the sansa clip headphone->line-in and have no complaints. (you may need to up the default line-in volume setting as its at 50% by default to avoid clipping with hot sources, i'd guess) Goes more than loud enough to annoy co-workers