@tzh21y,
Here is another recommendation for external clock,
http://www.grimmaudio.com/pro-products/master-clocks/cc2/
I think the guys suggesting to go with external DAC missed your point of exploring the external clock. I own the same player, If your goal is to improve SACD playback then external DAC will be of no use cause you cannot transmit SACD layer through a SPDIF output.
I would only explore external DAC if your goal is to improve CD playback which kinda defeats the purpose of owning a fabulous sounding SACD player from Marantz 😉