Low sound quality of high frequencies is often accompanied by the “sandy”, dirty sound of violins and cymbals, as well as the overly highlighted sibilants (whistling sounds in voice parts, like “s”, “sh”, and so on). If you use such systems for vocal mixing, you will have a hard time evaluating your work.
Cymbals distort the whole soundstage with the slaps resembling explosions of a strange white noise. Their right sound must have a certain delicacy, texture and tone. A good sound means a balanced combination of high frequencies with music, in which they don’t attract attention. As a mastering engineer, you have to be careful if you notice that monitors pair play high frequencies as a separate part of music.
Apart from everything else, special attention should be given to the monitors’ capability of playing high volumes without overload. Some models sound perfectly on low volume, but if you turn it up, they start yielding distorted sound.
Listen to the music (especially classic) during loud, complex parts – it shouldn’t crumble and have a gruff tone.
And finally, a necessary requirement is “dissipation” of monitors in sound stage. A pair of high quality monitors should unwrap music in the mix and master online studio environment, so you won’t even notice that sound actually comes from two wooden boxes.
In that case, performers have clear, tangible images, which appear between the monitors (if sound was recorded exactly that way). The instrument also shapes into such an image and has its own place in the sound stage without drifting anywhere when registers change. Music has to be open and transparent. Any fogginess, blackness or opacity is unacceptable.