It is not the loudness. It is the Dynamics . It is pretty easy to observe. You will feel like the headphones has no life to it, kind of like lifeless. When it is driven properly or something is able to drive it, the channel separations shall be crystal clear, details, resolutions, dynamics are all presenting. Now, if you want to know whether it can be driven better ? You need to try it from different amplifiers to hear it. When something is being driven Properly and optimized, the soundstage and layering, separations shall be vastly increased. Especially the punchy dynamics, it shall be stronger, denser and more energetic.
The 1Z is warmer, and I do think the 800S is a perfect synergy for the 1Z. The 800s is super Flat, huge soundstage that compliment the 1Z narrower soundstage, the vertical sense of rendering is nicely done on this combination. Trebles is dense and have the right kind of sparkling to it. I will be upgrading the cables for the 1Z soon.
Now, you can question me the next one, what is the Dynamic I keep talking about. It is the inner forces of an instrument that is making sound by the external forces applied on it, resonating within it housing, and creates a sound. If you are able to tell a healthy bass (resonates from the drum housing) and then the vibrating energy of it. I use example of the drums is because it is the easiest to understand and observe. When you can only hear Flat bass that you can not tell whether or not it is drum set 1 or 2 or 3 or where it is coming from, that is a headphones being under driven.
I hope that answered your question
The Dynamic is the punches and the energy that tell you different sizes of drums being played.
The fidelity is similar to the Dynamic, but take it a little further, it will show you the sound of that specific instrument from the beginning to the end of the plays and can not be confused by something else, even the same kind, but different size, or different plays. When the Fidelity is good, you will be able to pick and take a part virtually every instrument, note, play, from the beginning to the end of it. This usually only present in desktop of high quality system, and will cost an arm and a leg