If by imaging you mean soundstage, that is where headphones fall short, but its very tough for any speakers to match the clarity of headphones. I haven't compared the Behringers to other monitors directly in the same room (except for some old Alesis), but I would go for Mackie Mr or HR or JBL LSR monitors over Behringers. Mind you the Behringers are good for the price, but I don't think they are as good as say, some Adams.
As for a 8" woofer vs a 6", for near-field I would go with an 8". Not for its extra dynamic range, but for its extension, even if you will be using a sub. In a near-field setup, subwoofer localization is always much worse than a typical home audio living room setup in my experience. I think it's because when you are so close to the speakers, it's just easier to pick up the direction where all the sound is coming from. When the sub is say under the desk and the speakers are right in front of your face, the bass always sounds a disconnected, I have heard this in many instances even in some nice setups. So the reason why I would advise going with speakers with lower extension is so you can have a lower crossover. The lower you crossover the sub, the less localized it will be; the deeper the frequency, the less you are able to make out its origin.
By the way, what is your sub build going to be like, if you don't mind me askin'?
