You can consider Acoustic Energy Aego M. Nice speakers.
My solution was to use a pair of powered monitors (Tannoy Reveal 5a.) They're not too big for my desk, but might be for yours. The sound is incredible for the price (also you can add a sub later.) They're $500 new at sweetwater right now http://www.sweetwater.com/store/detail/RevealActive
so i'm assuming you can get a pair used in your price range. The imaging is silly good when i'm sitting at my desk working on the computer...vocals are dead center. Nearfield listening gets rid of so many problems that great sound becomes very affordable.
The Reveals take a long time to break in.
I really enjoy things that are, uh, a little different. For different in computer speakers, look no further than the actively tube amplified TAD-102 speakers from BizzyBee Audio. They originally sold for $799, but he has some closeouts for $99/pr. These may have a slight defect, but Paul will replace any defective units, so you will get a great sounding el84 tube amplified speaker for about $100. Check the closeout section at 2baudio.com .