Hi Andy,
I think considering your humble budget

...that ACTIVE studio monitors are the way to go for you.....unless you already own a decent amp....
Obviously your choices are somewhat limited, being in the $600 USD or under price category....but there are a few decent options out there....
Check out these...
Yorkville YSM-1P's
Behringer Truth B2031
Roland DS-90
M-Audio Studiophile SP-8B (not the 5B)
Tannoy Reveal $600 USD
Event 20/20 bias
There are also some new-ish JBL monitors within this budget - but I'm not familiar with the model numbers....
I've listened to all the above products myself in detail....and decided to buy
the Yorkville YSM-1P's, which I think are the best of the bunch and great value at $450 USD.......plus they are designed & made right here in Canada
Littledoggie is now gonna give me shit coz I said that - AGAIN -LOL!!!!!!!
But, speakers are so 'subjective

so ya gotta do your own checking out and decide for yourself.......
Happy hunting....
KEV