Thanks Rern! You probably won't be surprised that DAC selection is now sticking. It appeared to fix after the first set of cat commands i.e. without rebooting. Obviously due to the quantum effects of observing the system per Heisenberg.
Before reboot:
root@RuneAudio:~ # cat /srv/http/data/system/audio-aplayname
hifiberry-dacplus
root@RuneAudio:~ # cat /srv/http/data/system/audio-output
HiFiBerry DAC+
root@RuneAudio:~ # cat /boot/config.txt
force_turbo=1
gpu_mem=32
initramfs initramfs-linux.img followkernel
max_usb_current=1
disable_splash=1
disable_overscan=1
dtparam=audio=off
dtparam=i2s=on
dtoverlay=hifiberry-dacplus
After reboot:
root@RuneAudio:~ # cat /srv/http/data/system/audio-aplayname
hifiberry-dacplus
root@RuneAudio:~ # cat /srv/http/data/system/audio-output
HiFiBerry DAC+
root@RuneAudio:~ # cat /boot/config.txt
force_turbo=1
gpu_mem=32
initramfs initramfs-linux.img followkernel
max_usb_current=1
disable_splash=1
disable_overscan=1
dtparam=audio=off
dtparam=i2s=on
dtoverlay=hifiberry-dacplus