Just became a "happy" owner of Honor 9 with Oreo (188.8.131.522) on board and my Bose QC35 headset produces that low quality sound over bluetooth connection I could never ever expect from s smartphone, even a cheap one. To describe the sound - it is like a 64kbps MP3, crackling at low base and high notes. It's almost impossible to listen to such distorted sounding.
Bluetooth codec is always SBC despite of setting it to any other values available once Developer mode is enabled. So you don't have any impact on this aspect.
All of my previous phones were in a way more solid over Bluetooth than Honor 9 (Fly IQ440, Xiaomi mi4c, Xiaoimi Redmi 3, whatever!).
And how to fix it? I'm honestly considering returning it and picking a different one, listening to music through Bluetooth is actually probably the most common thing I use my phone for. Apart of that I don't see any related fixes in newer version of the firmware (184.108.40.2062).
If there is a chance if having it fixed, I'll be more than eager to provide as much information as possible either here or in a more appropriate place, just let me know which one.