Optical Out aptx hd receiver

[tomsag]

What is the standard Bluetooth protocol used by Phantom? Does it use BT APTX HD? If not, would this device improve sound quality?

Phantom only uses Apt X, which goes up to 16/48khz.  Optical fiber goes up to 24/48khz.  So you should see sound improvement with APTX HD that allows up to 24/48khz.  Correct me if I am wrong.

Well, Bluetooth is not a Hi-Fi standard at all. No chance to get anything like CD quality.
Stick to Upnp or buy Google Chromecast Audio and plug it in on optical input. Sounds fine, I have it. 
Consult this link for more info on BT: https://habr.com/en/post/456182/