I am using the USB audio output into the internal DAC of my NAD Amp.
Every time you I am turning of the Amp, the setting is lost and I have to stop and start Squeezeplayer manually again.
what exactly do you mean by losing the settings? Does anything change in the advanced options of Squeezelite when you turn off the USB DAC? Have you tested whether Squeezelite recovers after a few minutes after you have switched the DAC back on?
Can you post the debug log of the audioplayer page?
If your DAC has an on/off input then you could connect it to the gpio pins of the Pi. These can then be controlled by Squeezelite. I don’t see any other way to switch the DAC on and off together with Squeezelite.