most studio monitor speakers are no more "flat" than decent hi-fi speakers.
they are just more expensive and finished in different colors and materials.
even then, "flat" is not exactly what you probably want.
mostly the thing to watch out for with non-studio speakers are:
1. thermal problems... you run it too long, the woofer voice coil gets hot and saturates, making the sound change over time.
2. lack of resolution or detal. this is not about frequency response, but things like phase response, impulse response, and distortion, that are not easily measured and can't be fixed with EQ.
FWIW the performance of bona-fide "studio monitors" varies in these two categories nearly as much as the performance of regular hi-fi speakers.
There was a Sound on Sound article with an in-depth comparo of some popular studio monitors vs. hifi speakers for control room monitoring. very good. was online. Anyone got a link? I can't seem to find it. It'd be a good read on this topic.