No worries, let us know how it goes and what your friend was using for a dac along with amp. Hopefully we can figure this one out because the K3 specs at a minimum I would expect that the 400se should be a relatively pleasant experience rather than the description you gave it and the horrible turd fest that came out of my EPOS dac/amp. Plugging into my Galaxy S8+, I'm not a fan of how the 400se sounds either, "The lower mid range sounds muddy and untextured higher mids and treble sound veiled and everything feels very low resolution" was pretty closely fitting the description I had that you gave outside of needing more volume on the S8. Plugging into a Apple Dongle, it's not terrible but it sounds a bit rough compared to what I'm used to.

Hopefully by testing the headphone out on your friends gear will yield some positive feedback/findings to at least let us know if it is a gear thing or if the headphones sound signature is not a good fit, it's just a really tough call here because of the same-ish experiences I've had which back up your findings minus actually having a K3 to know for sure if its just a sound preference issue or the K3 is the issue here.

Hoping for the best.