I posted what works for me on hifiberry community, maybe can help also someone here.
I think there are 2 different issues that are easy to confuse:
1- the headphones jack cannot work in Volumio, SOLUTION: buy a 4€ RCA to jack adapter on Amazon, end of the story.
2- the DAC2Pro with Volumio after reboot doesn’t work anymore with “Alsa HW 2.0” error, SOLUTION: after the first configuration with everything working fine, switch off the pi, edit config.txt and insert command.
force_eeprom_read=0 restart the pi3.
I have a pi3 with DAC2Pro and in that condition finally everything works fine: jingle at boot, the led on the DCA2Pro turn on when playback, volyme control works, now it’s perfect!