I'm studying exactly that kind of facts at university here, in my country, so I think I can help a little bit...

Listening through headphones is called a binaural listening and not a stereo listening. So, if you hear a normal stereo CD through headphones you're always losing definition on stereo reproduction.
A stereo recording is meant to be reproduced through a stereo sound system, that's why all the mixs on the studios are made through near field monitoring.
Using a stereo sound system, there's the interaction between the sound reaching the left and right hear, but besides, using headphones that doesn't happen, creating a false sense of reality.

The monitors on a studio, or a even a home studio, should be like The Pro said: Studio monitors are designed to reproduce all frequencies within their range as evenly as possible.
When recording anything on a studio, remember that you have to get your hears used to your monitors, And always remember that the mix should be done not to sound perfect on those monitors but to sound average on every system. That's the difference betweem a good and a bad mix and that's why the studio technicians have to use this compromise on the final mix thinking about the results on a average system.
The pro, you're also right about playing the recording on the car, because as I said, the best mix is a mix that sound average on every sound system.

I think I answered the question...?

Luis Santos