No headphones are flat. None. Even if you spend ridiculous amounts of money on them. It's just not physically possible, AFAIK.
However, that doesn't mean headphones can't be useful as secondary references. The very fact that they do bring out different elements in the mix can be helpful. I wouldn't look for (or even expect) any headphones to match my main monitors because their value is that they
don't sound like my speakers.
In the $100-$150 range there are several widely-used models that should complement your KRKs well. Top of that list would be the ubiquitous Audio-Technica ATH-M50s. They're a little bass-heavy, but then your KRKs are a little bass-light so it might be a good pairing. Other candidates would be the Shure SRH940 or Beyerdynamic DT990.
But do yourself a favor and stay away from anything Bose.