I also have Beyer Amiron headphones but use Android. It should work with most portable usb dac's even with iPhone. I use an Audioquest Dragonfly Black and an iFi Nano. iFi has some new models like the Hip-Dac that looks interesting but if I would buy a new one I would go for the DragonFly Red or DragonFly Cobolt depending on how much I could spend.
https://darko.audio/2019/07/10-more-thoughts-on-the-audioquest-dragonfly-cobalt/
https://ifi-audio.com/products/hip-dac/
https://darko.audio/2019/07/10-more-thoughts-on-the-audioquest-dragonfly-cobalt/
https://ifi-audio.com/products/hip-dac/