Stop Volumio defaulting to onboard DAC [Help]

I have a Pi2 running in my car with Volumio 1.55. I have a uDAC USB DAC plugged into the Pi (and powered via USB) then onto the Amp. The Pi is also connected to a small WiFi router for wireless control from a tablet.

My issue is:
Approximately 70% of the time I turn on the car (and the Pi boots up), I have to go into the web GUI on a browser, Settings -> Playback and change audio output from sndrpihifiberry to “HP” which is what the USB DAC shows up as when connected.

I’ve commented out all the DACs in the etc/modules file, however from memory these are only for I2S DACs. I have a feeling the DAC doesn’t boot till after the Pi has done its initial scan, thus doesn’t show up at first, so the Pi defaults to sndrpihifiberry (which I assume is the default onboard DAC?).

Is there a way to disable the onboard DAC so this doesnt happen and I dont have to fiddle with Web GUIs while barreling down the highway? ;p