I'm currently using a M2 Tech high face DAC along with a pi3 B+ with volumio version 2.657 and have not any issues with popping or clicking. Some where I read the newer M2 Tech had issues. I'm using an older one about 4 years old.
Thx for the reply, I also have this DAC in another setup, indeed a great DAC. In the problematic setup it is the first one over there: http://www.newaudio.it/m2tech5.htm (no typo), which is also an older model.
Supplementary information about the setup: * From the DSP it goes directly to a Behringer DEQ2496 which also performs the DA-part; then to the amp and to the speakers. * The click/pop noise is proportional to the volume setup in the software mixer