I'd agree, the best thing to do would be to go to your local guitar center or sam ash, bring some CDs, and have listen.
The biggest problem I had with lower cost monitors was in the bass frequencies. Something with a small driver isn't going to be able to represent everything that low. Something to consider.
If I were spending $300, sadly, I'd probably go with the latest Behringer's monitors because they seem to have a good frequency response and seem to be like the Mackies.
Of course, all Behringer stuff seems to have a shelf life, a lot of what I have has broken and needed replacement. Something to think about.
Now, if you can get another $600 dollars, look at the Adam A7's....I just bought these, they are AMAZING!!!!