HiFiBerry DAC card - no audio and constantly reboots

having successfully used the Volumio player on my Pi for a week I decided to upgrade and add a HIFiBerry DAC card.
Alas, I get no sound at all when I switch the Output Device in Volumio to indicate I have a nice HiFiBerry DAC card, and as soon as I try to browse my NAS music folder, Volumio decides to reboot the Pi - every time!
Choosing the wrong card in Output Device (e.g. 503HTA Hybrid Tube Amp, the first in the list!) at least gives me sound but no volume control from within Volumio.
Is there an issue with Volumio recognising my DAC card, or something wrong with the card itself? How can I tell?
I have a Pi B and have added the P5 header pin as per HiFiBerry’s instructions.
Any guidance gratefully accepted with fixing this.
Many thanks in advance.

EDIT: Pi has temporarily stopped re-booting and now plays through HiFiBerry DAC, but no volume control via Volumio - volume options are either ‘None’ or ‘Software’ whereas used to be a 'Hardware option without the DAC installed. Changing option to Software from None causes Volumio to crash and Pi reboots itself.
Stuck in endless failure and reboot cycle…