I also had an volume problem with my new d400ex. (Very low volume) Only when playing USB PCM. This was not the case with BT or USB DSD (dop). After sending 1 email to hifi express.. got a direct reply from hifi express with a firmware update (from 3.11 out of the box) to 3.12. Now the volume problem is solved!

Nice work!
My first opinion after only 50 hours burn-in.. very nice dac, "much" more detail in the lower end in contrast to my topping d70s. Also an more natural 3D image. Still playing with filters and sound color.. (at the moment I like pcm filter: short sharp, and sound color: 3) Setup: direct d400ex XLR dac out - mogami - nad c298 - siltech new York - Von Schweikert E3 MKII.
In the attachment the original zip with the firmware update to 3.12. (use at your own risk) Don't forget to install the audio driver before the update.