I was personally looking at the MOTU M-Series for an upgrade for myself.. but I like having the an audio interface as a monitor controller with buttons to switch banks of speakers, mono, and dim.. which is REALLY hard to find on an audio interface.
The MOTU M2 Audio Interface seems to have very good amp for speakers/headphones, and very low-latency. (The M6 allows to switch speakers, but no mono/dim stuff.. so I’ve held off for now).
Let me know if you have questions about that, or open a new question (if you ever look into a dedicated amp for the headphones).
The benefits of an audio interface vs a headphone amp are the additional features like recording a microphone, plugging in speakers, in addition to being your sound card/headphone amp.