If I recall correctly you are playing back via DLNA. This means that the DLNA renderer is making some "decisions" about how to play things back. Many more decisions than a straight USB DAC would make. Try playing back locally as a test. Even if it's just the built in Mac audio. Use headphones or desktop speakers if you have to. Just as a test. This will tell you if it's a DLNA thing, or if it's something "deeper" to be investigated.
Brian.