The traditional analog 3.5mm headphone jack is being steadily removed from newer smartphones, leaving buyers with only two alternatives for listening to audio, using bluetooth earbuds, like the TWS600, and/or USB-C earphones, like the Hifiman RE400C, which deliver audio via the USB-C charging port, allowing you to enjoy similar sound quality you get over the useful, long-standing 3.5mm headphone jack.
Being USB-C wired, you won't have to worry about bluetooth codec support, lag or wireless range and, you get the benefit of near-lossless quality audio, which means being able to listen to CD-ripped audio and streaming audio, almost as the original source without the audio compression of bluetooth.
Inline the Hifiman RE400C earphone 1.33 meter long cable, there are a Y-splitter, which is made of thick hard plastic with the Hifiman brand wording printed on it in white. There is also an audio remote control box with a microphone built-in to it.
The remote control box is just 3cm long and has a compact, slimline design so, it does not weigh down the cable, which can sometimes be a problem with some wired earphones where the weight of remote box can pull the earphones out of the ears when running. The RE400 earphones are also lightweight, weighing just 15 grams so, they sit nicely in the ears.
The RE400C cable itself is not flat, which helps with tangle mess, but it is a strong, durable springy cable, thanks to the internal oxygen free copper wiring.
The remote control box is located on the right earphone cable, approximately 25cm down the right earphone so, the remote box dangles at neck level, which is a comfortable position to speak into the microphone during a call. The microphone doesn't have any noise cancellation but, it offers clear audio calls.

The RE400C remote box is made of plastic with a high gloss finish, while the three psychical buttons are low profile and made of rubber. The volume buttons have a "plus" and "minus" engraving on them, while the middle button (the play/pause button) has two tiny bumps that make the button easily identified from the volume buttons.
All three buttons are springy and responsive with the middle button being designed to carry out call functions (answer/reject/end call), as well as audio functions (play/pause and skip to next track). There is no support for skipping to the previous track. The buttons on the RE400C remote box aren't universal so, on some devices, all buttons may not work properly.
Both the left and right earphone cable are reinforced at the junction of the earphone housing via thick rubber strain reliefs that have the letters R and L clearly printed in white. There is also tiny bumps, which provide a more secure grab within the fingers.
The earphone housing shells seem to be made of two separate blocks of CNC machined aluminium with anodized finish and brushed aluminium finish on the back.
The Hifiman RE400C earphones have low 32 ohm impedance and only require 10mW of power to deliver full fidelity and loud volume, thanks to the 102dB high headphone sensitivity of the 8.5mm speaker drivers. You won't need a headphone amplifier but, if you do plan to connect the RE400C to an external DAC, be mindful of the 30mW maximum power rating.
The sound quality is good with clear, accurate highs even at max volume but not as loud and artifact-free as with the RE600S V2 wired earphones, which have an EMI shielded wiring.
There are two types of silicone ear tips included, black ones and white ones. The black ear tips are short and stocky, while the white ear tips are thinner and longer, providing a more secure and more comfortable seal on the ear canal. The black ear tips are made of slightly thicker rubber so, they aren't as comfortable as the white ones.
Included also with the RE400C USB-C earphones are a spare set of filters, which soften the highs, while slightly increasing the bass response, which is also aided by the tiny vent holes drilled on the driver housing. That said, to really make the most of the bass, you really want to use foam ear tips to get the best possible seal.