It seems to be some kind of DSP trick. I have seen this in other subs from Monoprice. I have spoken to the engineer of these subs about this, and he isn't giving away the formula to the secret sauce.
According to Floyd Toole and some other experts, group delay is so subtle (Refer to 3rd paragraph and below):
Taken from Floyd Toole's book entitled "Sound Reproduction 3rd Edition" :
"Many investigators over many years attempted to determine whether phase shift mattered to sound quality. In every case it has been shown that, if it is audible, it is a subtle effect, most easily heard through headphones or in an anechoic chamber, using carefully chosen or contrived signals.
There is quite general agreement that with music, reproduced through loudspeakers in normally reflective rooms, phase shift is substantially or completely inaudible. When it has been audible as a difference, when it is switched in and out, it is not clear that listeners had a preference.
Others looked at the audibility of group delay. They found that the detection threshold is in the range of 1.6 to 2 ms, and more in reflective spaces.These numbers are not exceeded by normal domestic and monitor loudspeakers.
Lipschitz et al. (1982) conclude: “All of the effects described can reasonably be classified as subtle. We are not, in our present state of knowledge, advocating that phase linear transducers are a requirement for high-quality sound reproduction.” Greenfield and Hawksford (1990) observe that phase effects in rooms are “very subtle effects indeed,” and seem mostly to be spatial rather than timbral. As to whether phase corrections are needed, without a phase correct recording process, any listener opinions are of personal preference, not the recognition of “accurate” reproduction."
