I've successfully used the JVC HAS500 to great results.  Not only for mixing but as well for mastering.   I find the S500 to offer response very similar to what you'd expect from a decent pair of monitors like a set of Quested.   Mind you the room acoustics doesn't come into play which...

Just grabbed a pair HAS500 off ebay "slightly used for 4 hours" for $57 shipped. :) I can't wear to burn mine in, they also came with a pair of AT pleathers so I'm all set to try them out! I chose these over the Sennheissers of the world etc...I'm a professional studio engineer so I'm...