Thanks for the graphics, no color at all.
Humm, from the time I had the Dragonfly, my iPhone had a lightning port, that restricts a lot the power to peripherals.
I noticed (with the go link) a remarkable change on iPhone 15 pro max with USB port (which is class compliant 3.0).
But this doesn’t explain the huge difference within the Zen Signature and the Focusrite Scarlett. I connected both to the same source, barely no difference on signal strength, but the sound is quite different (Ifi has way more coloration and less clarity).
Also bought a Wiim Pro streamer and the sound is again different from Focusrite and Ifi. In this case very mid bass focused and highs are reinforced also.
Wiim pro offers the possibility of changing filters, that changes also slightly the sound perception but still is minor difference compared with the streamer-dac signature sound.
Probably is because some other reason than frequency response, I guess that FR is not the whole history of how an electro acoustic device will sound…