Looking for open-back cans

Obviously, by the title I'm looking for a pair of open-back headphones. Too be more specific I'd like them to be at the most about $250, have more of a flat frequency response, have a lower impedance due to my lack of a proper amp, although I'm not ruling of getting an amp too, and be over-ear. At the moment I have the ATH-M50's with a ModMic 4.0. As my experience with Audio-Technica was been great so far, I've taken an interest in the ATH-AD700X/900X. Can anyone vouch for the quality of those headphones? Anyway, thanks for the consideration and time taken to answer this. 

AKG have a few semi-open headphones that are on massive sale on amazon. I have a pair of AKG K99 Perception headphones and they sound lovely and don't require an AMP. Flat, large sound stage and are now $50 instead of $150 or so that I paid for them.

I think the AKG headphones that are a step up from these are also on sale (but require an AMP as they are 55 ohms).

Maybe you should consider buying one of these together with an DAC/AMP. That way you will see an improvement even in your current headphones.

Thanks, I've heard of AKG but never really took the time to check them out. Would the Mayflower Objective 2 DAC + amp be a good idea? From what I heard, it outperforms high end tube amps.