Volumio Primo has an internal DSP working on analog outputs.

DSP improves the digital audio signal conversion, especially allowing certain types of signals to pass and block the rest.

2 filters options are available

Click here to know more about FIR filters:

FIR Filter Basics - dspGuru

Click here to know more about IIR filters:

IIR Filter Basics - dspGuru

Finally, after learning about filters, always trust your ears and your soundsystem..