Sennheiser 595's are good headphones if you don't have a dedicated headphone amp. However, after switching to the 650's, they tend to have a little too much emphasis on the mid-high end. 650's are more accurate in all forms of presentation.
If you're not willing to pay for a separate headphone amp, then don't get the 600's or 650's. The 650's are revealing enough where you'll want to upgrade your source, dac, and amp...so it's a bad road :D.
Open headphones are "open" they mean that there is nothing separating your ear (and the speakers) from the outside world. So people can hear you and you can hear others.
I encourage you to look on head-fi.org. A huge wealth of information over there...so start searching
Some models you could look into would be Grado MS2i's (about your price range and don't need an amp) and Denon D2000's. D5000's are the same thing except with a few tweaks...no change in the driver. If you look into the Denon D2000's, you can do a mod called the markl mod.
That should get you started!
