Any excellent DAC will not benefit from an external clock. The good ones, like the Benchmark DAC3 reclock S/PDIF themselves, so the only jitter is internal.
Why buy a $700 reclocker when you can get $700 or cheaper DAC with external jitter reduction?
For instance, the Schiit Modi 2 showed improvement when the Empirical Audio $700 reclocker was added, but pretty much no difference when used with the Topping DX7s ($500 DAC+headphone amp).
Going digital out of your Marantz to say the SMSL SU-8 would give you similar results to using an external reclocker, in that the jitter from both would be well below audible levels.