very flat response on axis, well controlled off axis ie good dispersion and low distortion when the speaker is used within its limits. These are characteristics of all good speakers "studio" or home.
Flat speakers dont sound good to most folks with normal hearing. If your ears already have a dip in response in the mids then flat might sound better to you.
The NS10s arent flat. Nowhere near it. I've seen many other studio monitors that are not flat either.
Studio monitors are for people that enjoy reading specs and measurements. For most audiophiles we care more about how it sounds so buying a studio monitor would be a mistake. I'd say make flat speakers illegal and make custom tuning the law!