I would seriously consider you find a local store thats sells pro audio gear and see exactly what you can get for your budget, there is plenty of choice at this sort of level. I would aim to try and find a minimum of a 6inch driver, as most smaller drivers can struggle with really low frequencies. See what you can get for you money, IMO you should put a few different brands on your list and try to test as many as you can, I would recommend ADAM as a brand having used the A7x for a while.
Also consider brands like KRK Tannoy and Mackie in your range if you can. I know in the UK its hard to get a good listening session without prior notice so if you have a local supplier give them a call in advance and explain your budget and see if they can give you an audition.
Once you go into the $600+ market you can get a pretty good set of monitors, and they are all pretty good value IMO. There are very few "bad" monitors once you start spending this sort of money, and you really do get what you pay for.