I'm not looking to inform you because I don't presume to know more than you, and I certainly don't want to insult you, but I'm going to break things down as simply as I can because I'd like to make sure I'm being understood.I can make many claims and insist that they are true... Until someone comes up with evidence to the contrary.
I'll start with loudness because we can agree that it's a measurable, objective phenomenon. If you take a sound and reduce it's volume without touching it's frequency balance and timing, you'll perceive that sound to be more distant. Louder also music sounds "better". Therefore, loudness has an audible effect on sound independent of tonal balance.
Let's say that on a given headphone, Frequency A is supposed to be expressed at 2x the loudness of Frequency B and that in the first half-second of a song, the loudness of frequency A in the recording goes from 0 to 15 to 25 to 50 to 100 to 0. The headphone might express these loudness levels as 0, 20, 20, 60, 100, 0. Frequency A is always being played at 2x the loudness of Frequency B, meaning tonal balance is maintained throughout playback, but the music will be perceived differently as a result. This is what I'm referring to as dynamics.