When I connect my iPhone to the USB dac at the back (recently bought a dedicated cable) there are no pops and clicks, so this confirms that my Windows laptop is the popping and clicking culprit.
But I am not really hearing the supposed better sound using the USB DAC at the back instead of the port on the front....which is fine, because the sound is just wonderful.
True, SPDIF or coaxical connection is just a tad better overall, as it is with my other DACs in comparison with USB, but the difference is less obvious with this Marantz.
What I have noticed as well is that the amp's gain setting at medium seems to result in a better sound overall, with all my headphones.
And for what it is worth, the amp/DAC is dead quiet. No noise, no incidental high pitched swirls of jittery thingies. Pitch black. No need for an IFi iPurifier.
To be frank, the jittery stuff never bothered me much with my previous DACs or the headphones output of my laptop, but it is nice to hear...erm it is nice to not hear any of that any more.
Taking all this in account, this Marantz must be one the best bargains in recent hifi history, and I got a unit in mint condition for only half the price!