I must say right away that my opinion is largely the opinion of an amateur, albeit with some experience with sound, because my usual control is based on a monitor speaker system, and not on headphones, although during recording I have been using old ones closed akg 271 studio for about 10 years. .
Therefore, I can only compare with them, and with my wishes, of course.
bass and sub-bass
First, in connection with the transition to home mode of operation due to covid-19, I was looking for several devices for monitoring various purposes - from recording to mixing. And HD9 pleased me in this sense. What I could not catch in my old AKGs, I found in this Presonus. This is primarily the detail of the bass and sub-bass. These headphones really made it easier to work out this part of the spectrum in limited conditions, although it is heard very redundant.
fit and comfort
In general, I cannot complain about their fit, but their ear pads are oval, somewhat elongated, and I am used to putting the bow of my akg on my neck - this saves movement and round shells allow you to do this. But with Presonus this will not work. Well, the cable is not detachable, although I don't need it now.
Another drawback (again - subjectively and in comparison) - the ears warm up pretty quickly. They don't sweat, but they are hot. When you have to work outside in the cold, the leatherette finish quickly sets in.
I think there should be no complaints about the nimbleness of the design - after all, any functionality has its drawbacks. You get used to it, and after two weeks of use, the monolithic rigidity of the same akg begins to annoy me.
not ideal, but wonder for a work in price
Summing up, I can say that I am satisfied with the purchase of these headphones. Yes, they really lack a section from 500 to 700 or even 800 hertz, and the upper middle is a little decibel by 3 or 4, but dynamic intelligibility will allow you to adjust the equalization to your preferences with almost no loss in the clarity of attacks. Well, in fairness, I must say that any monitor headphones are a tool for solving a narrow range of problems, and physics itself will not allow us to choose one model for all occasions. So for their area, these headphones are very good, especially for such a price.