Oh I get you now, @last_lemming
. The Marantz is feeding the contents of your USB drive via coax to the NAD. Like Erik said, for coax, the clock is embedded in the SPDIF data stream from the Marantz. Here the source is the clock master and this signal is synchronous. Folks used to sell SPDIF reclocking devices on the principle that the clocks in most original SPDIF sources (e.g. your Marantz) were bad (i.e. causing high jitter) and the doohickey reclocker would be better....
Now, you could stick the same files on a laptop and plug the laptop into the NAD’s USB input, but this time it’s async cause the DAC controls the clock. Despite this improvement, folks still try to improve the quality of USB sources with new doohickeys (that I mentioned in previous post) on the basis that computers have a lot of electrical noise that come into the DAC.
This is an old article from wavelength but still explains the concept of async USB http://www.usbdacs.com/Concept/Concept.html
You can also read product briefs on things like the iFi iUSB3, Uptone USB Regen. Folks have also now moved to Network sources and transmission devices like your Marantz all-in-one, Auralic G1, SotM, DCS Network Bridge, etc.
I don’t think you need a relocker in either case cause of pitiful diminishing returns! Expect strong rebuttals from those hawking the doohickeys! Note I’m not saying none of these devices can improve the sound, I’m saying they’re usually expensive to the point of (all added up) where you could get a much better DAC / room treatment / Scotch / beer money.