This goes absolutely against the real-world experience of professional mixing and mastering engineers, people who make their livings on "accurate" reproduction of music. Speakers are not accurate, period. Anyone who has been both a musician and a mixing engineer (like me) will tell you this. The idea of "accurate" response from audio systems is magical thinking.

The mixing and mastering engineers aren't trying to make some perfect reproduction of natural sound. They're trying to make records that sound as good as possible on as many different kinds of reproduction systems as possible - not just "perfect" audiophile systems, but car speakers, iPods, etc. As such, rather than going for accurate speakers, mix engineers rely on speakers that they know very well, so they can predict results elsewhere more easily.

The most popular professional mixing speaker is the Yamaha NS-10. It's not "accurate". It doesn't even pretend to be accurate. In fact, it has a pronounced peak and significant harshness around 2khz, right in the most sensitive area for vocals and midrange melodic instruments. Why use it? Because if you can make it sound good on the NS-10, it'll sound good anywhere. Likewise, the second most popular speaker is the Auratone, a single driver with limited frequency range. The Auratone has two advantages. First, it reflects the limited construction of many real-world speakers. Second, because it lacks a crossover, there's no phase weirdness in the midrange, so it's actually very pure at the most musically critical points. Deep bass and sizzly highs aren't important. Midrange is important, and Auratones are brutally honest at that, more than speakers costing orders of magnitude more.

Are they? I haven't had the chance to compare two monitors in the same room, but as far as I know no two are alike, and none has ever achieved a perfectly flat response curve. Take also high-end headphones, where it's way easier to have a similar acoustic profile, yet any two models with similar characteristics (on paper) are very easy to tell apart.

There's no such thing as a perfectly flat response, but that's not what you need: what you need is response flat enough that it is within human ear resolution.

(As far as I remember, headphones are actually harder to get precise response curves due to the interaction with the skull and precise physiology of the listener, but I may be mistaken in that.)

The problem with any claim from people that they can "easily" tell two pairs of headphones apart or two pairs of monitors apart is that most of the time these claims are not scientifically validated. There are numerous biases and gotchas involved in measuring audio fidelity, and one must be aware of these when designing experiments. (And the whole "audiophile" industry is based on the idea of selling snake oil technology to people with fat wallets and who think that they have better ears than anyone, and they go to great lengths towards denying the science.)

Likewise, it's very easy to write off actual observation as "bias" when the test isn't "scientific". That's one of my big gripes with what I thin of as the right wing of the hi-fi industry... the idea that human experience can be reduced to measurable and mechanically reproduceable observation in all cases. This reaction leads to "blame the observer" rather than questioning whether the scientist actually understands the problem ___domain or the question they're asking sufficiently well. It's every bit as emotional and reactionary as the "left wing" hi-fi subjectivists. But at least the subjectivists are owning up to their magical thinking.