When monitoring loudspeakers are designed with as small input-to-output delay as possible, the delay typically increases towards low frequencies. This is true for both fully analogue loudspeaker designs (passive and active) as well as for designs using digital signal processing (DSP). The audibility of the delay depends on how much the delay is different between frequency ranges and how wide these ranges are. If the delay is different in a narrow frequency range, this is led audible to humans than when the delay is different across wider range of frequencies. Human listeners are the most sensitive to delay variations in the mid-range of frequencies (about 500 Hz to 4 kHz). Studies show that it is easier to detect delay variation when listening over headphones compared to listening to loudspeakers. For headphone listening, the detection limit for the most sensitive persons (also called the Just Noticeable Threshold) typically lies in the area of 0.5 to 2 ms in the most sensitive frequency range. Genelec monitor designs push the delay variation in the monitor passband below the limit of the detection limit. 

For more information of the detection limit for delay variation, please look at

https://www.aalto.fi/en/news/the-human-ear-detects-half-a-millisecond-delay-in-sound

https://ieeexplore.ieee.org/document/9450008