I have a Sony 4k player has 2 HDMI outs one with video and audio the other just audio. I send the just audio to the HD621 that i picked up from eBay for a 100.00 about six months ago. Worked out great!
I can now listen to the more modern codecs from blu-ray/4k discs.

Dolby TrueHD, DTS master, etc. One big surprise and I don't know why but redbook CD's sound better than they ever have. The HD621 is worth it just for that improvement.

These can be had for a song smile these days. Highly recommended!

Ron