Hi, I am reopening this old discussion to update it with a recent experience of mine.
In the replies, the "headphones" hypothesis suggested by the seller was practically ruled out, but I believe it is worth considering.
The possibility that headphones can replace monitors in mixing is normally denied; however, with a bit of experience and many mistakes, I realized that if you do not have a suitable environment (normalized, as Simone writes), no nearfield monitors will suffice.
My DAW is in a large L-shaped room, without doors and with an open staircase leading to the floor above; it is practically impossible to soundproof, so I had to resign myself to the idea of looking for reference headphones. So, I tracked down ALL the available charts on the web (frequency responses, harmonic distortion, and so on), comparing an infinite number of brands and models to find the most linear open-back headphone possible; I bought it and set the monitors aside. Among other things, I only trusted technical data because I didn't have the chance to listen to them beforehand; and I was right, because only then did I realize that I had never managed to do a decent mix before, neither in my house nor in those of friends equipped with the best monitors, 16-inch cones, absorbent panels, and everything else. After all, below 50 Hz you don't hear anything anyway, and if you put it in a house, a subwoofer blasts random rumbles (not to mention potential problems with neighbors, roommates, etc.). Incidentally, in the most common listening devices, the perception of sub-bass is a psychoacoustic phenomenon. The only reason I resort to my old monitors is to watch the "silent" movement of the cone, precisely because they are an indicator of low frequencies (meaning they "float," even if they don't reproduce them): so I cut at 20 Hz and don't think about it anymore.
So, at least as far as I am concerned, I had to conclude that a pair of linear reference headphones is worth much more than excellent monitors placed in the wrong environment (and they cost much less!). And that if I have to do "serious" work, I must fine-tune it in a professional environment, i.e., one designed for that purpose.
Naturally, Simone is right when he writes that quality can also be achieved at home; however, one must be able to choose the right room (I couldn't) and spend quite a lot to make it a very "clean" environment. Objectively, this is not always possible.