Hey, So XEQ works beautifully for me. Make sure you have audio-compatible mode enabled in the settings.
I believe the issue with the P9 is it is making every song a new session, and the multi-session is causing issues. Poweramp eq has a "fix" for multisession, but it does not work or is out dated. XEQ works wonderfully for people looking for an alternative since the developer here doesn't seem to care. I recommend trying XEQ, it will work with Android Auto as well as headphones.