In my opinion, it makes the HD 58X sound dynamically impressive, and with a tall and wide soundstage. Music sounds around the head, not inside or just in front.
Imagine a DAC/amp combo showing you a single piece of paper with all the track elements drawn on it. It sounds good and clean, but image-wise, aurally flat . Every sound is coming pretty much from the same plane. Lots are like that.
Now imagine a DAC/amp combo showing you every track element on its own piece of paper floating at different heights over one big piece of paper. The Audient iD14 is like that. It's a joy to listen to music with it, because whatever is in the track and however it is, you will hear it clearly. While it's not a "bright" amp, no detail is left behind. If it's there, your ears won't miss it.
EVERY track in an album sounds different through it. You can really hear the difference in microphones, compressors, effects, etc. It has one of the best headphone outputs I have ever heard in my life for not being a dedicated audiophile headphone amplifier, and it also records as good as it sounds. For $299, it's one of those hidden bargains. God bless!