When auditioning a1 lumin with the integral T+A no pop sound was pesent and the volume of lumin was at 100.
I also tested two units of Lumin with DSD upsampling in a normal configuration with pre-amp, and there are only very tiny click sounds if I stand very close to the speakers. I believe what you experienced is that the tiny click sound is amplified for many times to become a loud pop, because there is no pre-amp to attenuate the signal. If you are worried that your unit may have a hardware issue, see if you can go to a Lumin dealer and compare it with another unit.