I have solved the same problem with my new SONY SBH60 BT headset now. When BT device support both audio in/out, iOS playback sample frequency is reduced by two-way “phone” codec in the BT device. Default mode of my SBH60 is a “headset” and his internal microphone is active. In this mode, BT headset works with “phone” low sample frequency codec (in my case about 16kHz) and music playback quality is horrible!
When BT headset is in one-way headphones mode (internal microphone on the headset is inactive), BT headset-headphones works on high sample frequency codec (in my case 44kHz) and sound in the headphones is amazing.
Most of iOS apps have a perfect sound in the BT headset as default, because these apps do not use audio-in, but only the audio-out and therefore only applies headphones mode and codec with high sample rate = perfect sound. But Cubasis uses audio-in and audio-out together.
In Cubasis can be mode set there: Setup / Audio / Bluetooth - Airplay. When the switch is “on”, the sound is perfect from my SBH60. But microphone is disabled, because headset works as headphones (audio-out device only). When the switch is “off”, paired SBH60 is automatically used for recording too (audio-out/in device) and playback is horrible, because Cubasis uses SBH60 in two-way mode with low frequency codec.
Solution: For quality audio out on BT headset must be this switch “on” (Setup > Audio > Bluetooth / Airplay > Bluetooth / Airplay Output) When switched “on”, Input device is set to None.
Problems for improvements in future versions of Cubasis:
Essential and big inconvenience is that Cubasis can’t remember this setting and must be set manualy every time when Cubasis is started. Too uncomfortable!
If this switch is “on” (for quality audio-out plyaback) and audio recording is started on some audio track, Cubasis automatically switch this to “off” and uses BT headset for recording. That’s fine, but when recording is finished, Cubasis does not set the switch to its original state. Unfortunately switch remains “off” and must be set to “on” manually again. Too uncomfortable!
I hope, that this issues will be fixed in some next version od Cubasis.
Described findings are for the iPad Air 2 and SONY SBH60. But I am convinced, that this also applies to many other BT headsets. For information and advice on this issue, thanks mainly to AudioShare app support!