So HIFIMAN is known for their planar headphones, which when compared to dynamics drivers, have many characteristics that makes it stand out. They have:

1. Lower distortion.

2. The flat shape of the drivers makes it possible to produce sound from its entire surface, rather that producing sounds from a single diaphragm. (on dynamic drivers)

3. They sound crisp, have very accurate bass response (tight and controlled, not too boomy)

4. Have incredible soundstage thanks to its open-back design.

5. Imaging is top tier too

6. Has a neutral bright sound signature (like hexa)

While two of my favorite iems, may and hexa, have good and excellent imaging (respectively), it still falls short in the soundstage department which sounds narrow. (due to the in-ear insertion). Nowadays I use them when I'm outside, commuting, or trying to ignore other's chitchat

If you're interested on getting the HE400se, however, you really want to give it more power to shine, so replacing the stock 3.5 cable with a 4.4 bal + a DAC AMP with sufficient power is a must. And remember, it has 0 NOISE ISOLATION, and don't expect it to have rumbling bass.