Chrisjohn you'd need to tell what DAC you're using because "changing led colour based on quality" is too vague. My understanding would be it could react to higher than 48kHz sample rate, or perhaps bit depth etc.
The thing is connecting DAC to USB - with USB Audio mode and Spotify, you're getting always resampled 48kHz signal which comes from the Dudu android mixer.
As your DAC probably does the Christmas colour lights only when sampling rate is > 48kHz, that's why you don't see them at all.
If you don't use USB Audio bit perfect with other apps like UAPP, you are actually degrading the sound quality.
Because you are sending digital audio to DAC, which converts it to analogue, then you feed analogue (with possible noise, as these are analogue lines in car environment) to your DSP, and DSP converts it back to digital. And, at the end your DSP converts it again to analogue for amplifiers.
So, as you can see, you have 3 conversions.
If you use spdif (which is same quality as usb audio if you don't use bit perfect direct from some apps - spotify doesn't use it) and feed it directly to DSP, you have only 1 conversion in the chain - your DSP converting to analogue for amplifiers. All the rest is done in digital domain. Arguably DA and AD conversions bring most of the distortion and noise to the signal.
Unless of course you have DAC with spdif out, then it's all good (providing it doesn't do spdif from analogue conversion).
