LDAC is still the best in terms of audio quality using bluetooth, if you are an audiophile, you need to stick with LDAC.
Not getting into a lot of details, but in summary:
Bitrate | Max Bit-Deph | Max Sample Rate
SBC: 328 kbps | 16-bit | 48 kHz
aptX: 352 kbps | 16-bit | 48 kHz
aptX HD: 576 kbps | 24-bit | 48 kHz
aptX Adaptive*: 279 - 420 kbps | 24-bit | 48 kHz
LDAC: 330 / 660 / 990 kbps | 24-bit | 96 kHz
* uses better compression ratios when compared to aptX HD which means lower bitrates to produce better/same quality as aptX HD
I noticed that with the Software update from early January 2019 for my Note 9, I could no longer use LDAC. Previously I can enable LDAC on Note9 using my Sony XM3 Headphones. Bluetooth codec always reverts back to SBC when choosing LDAC or even AptX. Same issue with the Android P update that i was able to install yesterday. I was hoping that the Bluetooth codec issue would be fixed with the Android P but it seems that the issue persists. Not sure what to do or how to fix this.
Any news? It's ridiculous since AptX Adaptive should be supported by default on Android Pie, but I don't see AptX HD or AptX adaptive listed under codecs after the Pie update. "Samsung HD" has been removed as well. I don't notice any issues with default codecs though - my MDR-1000X defaults to LDAC (although I do have to change the quality mode) and other devices to AptX or AAC. Could it be that the Exynos devices can't support it so they decided to drop it from both?
On a more positive note, Bluetooth latency in games is almost gone in Android Pie!
Galaxy Note9 owner here, just updated to Pie. I tried changing Bluetooth audio codec to LDAC but it changes back to SBC the moment I close developer mode. Is it normal? Same goes for any changes I made in Audio Sample Rate, Bits Per Sample and playback quality for LDAC. Is there a way to save changes or will they be saved only when Bluetooth device is connected?
The phone chooses what codec to use automatically so assuming you are using a pair of Sony branded Bluetooth headphones that are LDAC compatible that's the codec it will use. You can change it manually but only to something that both devices use and only when the headphones are connected.
So, as the original poster of this thread, I can announce that Pie has finally arrived on my Note 9 and as far as my original issue goes.... no noticeable change. Audio sample rate still defaults to 44.1 kHz. Bits per sample still defaults to 16 bits/sample. The same as before the update which has supposedly introduced aptx Adaptive. I can manually change each setting but as soon Developer Options is closed they appear to revert back to the original settings. This happens when listening to Master quality tracks on Tidal (MQA tracks are now available through the Android app) which are supposed to be 96kHz, 24 bits/sample. I realise that both aptx HD and aptx Adaptive have a sample rate of 48 kHz but they also both output at 24 bits/sample so why does my phone still default to 16 bits/sample? Taking all of the above into account and connecting to my Bowers & Wilkins PX headphones I should surely be seeing the audio codec defaulting to aptx at 48 kHz, 24 bits/sample? The photos below show what is actually happening. LDAC quality is different as I manually changed it but as LDAC is a Sony codec I don't think it's relevant and the sound quality wasn't any different when it was changed. To say I am disappointed is an understatement as I actually thought that Samsung were listening to their customers.
Bluetooth settings in dev mode will not change based on what app or file you play but based on what headphones you connect. I have just received my Sony WH-1000XM3 which support almost all modern codecs. Tried it with Note9 and Onkyo DP-X1A DAP. Onkyo switch to AptX HD automatically the second I connected my Sony WH-1000XM3. Now with Note9 you need to take another step. In Bluetooth settings when you click on settings of connected Bluetooth device it showed for my Sony WH-1000XM3 - LDAC which is best Bluetooth codec which was supported by both my Sony WH-1000XM3 AND Galaxy Note9. And after that it sticked no matter what I did, reconnected and connected device back. Also all settings in Bluetooth Dev mode went on maximum because LDAC supports it: 96kHz/32bits.
If I would disable LDAC in Bluetooth settings of connected Devices, Note9 would automatically choose second best which is AptX (not AptX HD because Note9 doesn't support it)
I suggest you first check and enable the codec for your B&W WHEN they are connected over the Bluetooth. Go to Bluetooth connected devices and go to settings of connected B&W. If it will offer you AptX or anything better that your Note9 supports it will stick.
You're using a 3rd party external DAC so this isn't really a fair comparison. My headphones are aptx HD compatible. I was told aptx Adaptive was backwards compatible and was being incorporated into the new OS update, however it only shows up as aptx and nothing has changed from before I submitted the post. If there was a way to confirm that Adaptive was actually being used then I'd accept that it was working but as it stands it looks like it's exactly the same as it was on Android Oreo.