No such thing as flat response
There is no such thing as a speaker with a flat response - neither in theory (maths) nor in practice (electronics + mechanics) such a speaker can be built.
The term is often used like so:
Flat response, 60Hz - 20kHz, ± 5dB @ 100 dBSPL.
Which means that within the frequency range given (60Hz - 20kHz), when test signal was played at the given level (100 dBSPL), no frequency was 5dB louder or softer from the given level.
So 5dB is your 'error margin' and normally the lower the better (a figure of ±3dB is generally excellent, ±5dB considered good, ±10dB is little professional.
Consumer vs pro speakers
What is 'correct' is that consumer speakers are built to please listeners, while pro speakers are (more often than not) built to be 'true'. In other words, consumer speakers are 'coloured', while pro speaker strive not to be such.
A better 'flat' response is one criteria that pro manufactures try to improve. But there are many others - distortion levels, bass response, bass 'tightness' (for how long bass appears at the output, once no such exist in the input), efficiency, etc.
Pro manufacturers typically invest a lot of research to improve the various criteria and often use original component, designs and materials. Consumer speakers are often an assembly of ready-made components highly tweaked for cost-impression.
No way to compare
The spec sheets provided by manufacturers are hardly a thing to consult - not only these are lacking accurate specifications, but much depends on the test conditions and measurements chosen - manufacturers can easily 'cheat' by presenting the best tests and it is impractical to give specs that cover all frequencies at all levels with a large variety of test signals.
Anyhow, most people compare by ear. What is important to consider is that not all people work on the same type of music and thus there isn't 'one-speaker-fits-all'.
My experience shows that people just get used to their speakers and you have to remember that enormous amount of songs where mixed on the Yamaha NS10s, which were never intended for professional use and by all means have rubbish specs and a non-impressive sound.
What to look for
Although a matter of opinion, there are a few thing you may wish to look into:
- Are the speakers ported or sealed-enclosure - sadly due to cost tradeoffs, there aren't as many sealed-enclosure monitors as there are ported ones. Sealed enclosure monitors generally produce a more accurate and tight bass response, but the price you pay is often less bass.
- Frequency range / 'flatness' - the very measure shown above - how low the bass goes and what's the error margin (a higher error margin will also mean a lower frequency range; a 70Hz - 20kHz ± 5dB could be presented as 50Hz - 20kHz ± 10dB.
At the end of the day
- The more speakers cost the better their quality should be.
- You should make your decision based on listening to various monitors.
- It doesn't matter much - people just get used to their pair.