Not sure if this will help you but I run the hires radio app from iPhone through camera adapter then into Ifi hip dac headphone Jack out into aux in on my Lexus. My Lexus has the Mark Levinson system and it sounds great. Full body with kick to stomach bass. The hip dac has a great amp section snd a loudness switch that steps it up. You might be hearing what you are as the first out put is a 2 watt class d amp from the iPhone. Your best option might be to upgrade from phone to mid level streamer.
Hi Res from iPhone vs AppleTV sound
Not well versed in digital music.
I’m retiring my iPhone 7+ and thought I’d get a camera connection kit and usb cable and run the phone as a high res playback direct to my DAC.
it’s running into a Schiit Modeus.
My AppleTV doesn’t do hi res, but the signal goes via HDMI to my tv which passes the audio to Optical out and into the optical input on the Modeus.
I was A/B’ing then and the hi res via usb is noticeably thinner whereas the ATV. Is full.
Maybe there’s more to it?