Agree with Folkfreak and I am afraid not with mzkmxcv. Noting that like Folkfreak mine is based on actual experience of owning dCS and other equipment capable of external clock use such as Aurender W20 and Trinnov Altitude 32.
First point to OP tzh21y - dCS individual one box units with internal master clocks that are already of a good standard - Puccini and Rossini benefit audibly from an external clock and whilst I would note that better level external clocks give still better level performance the Puccini word clock (no longer made but readily available) whilst the cheapest of the more recent dCS clocks managed to:
1. Improve the performance of the dCS Puccini which I then bought - noting now (I can expand on this) that clock cables make a difference but even the uner £20 type that dcs supplies free (and I will happily post you one for nothing if you get a clock without one) are good enough to show the difference.
2.[When I had a dCS Scarlatti before I had a Vivaldi system] I was attending a small show in the UK where the demonstrator (who was using these for a sorce for his equipment not 'selling' them) had not fully understood the importance of the clock sync between multiple dCS units - in this case a Vivaldi Transport and Vivaldi Dac (which could have been clocked with one as master using the same leads) the demonstrator did not have a Vivaldi clock, but did have a Puccini U-Clock. I asked (and assisted) in this being installed to the two far more expensive units. The performance of the Vivaldi two units system was improved by the synchronisation of the external clock. Noting that as mzkmxcv I believe was prepared to acknowledged multiple unit synching does provide a benefit, it was not possible in that show to test whether synching from dac as master to the trasnport would have matched the U-Clock, but the performance improvement was such that two attendees who were Nagra owners that had come to hear dCS stated it made the difference between them forgetting the idea of dCS and getting a proper demonstration with an appropriate clock. The comment re smearing is actually relevant because whilst I would hardly have called the vivaldi performance without the clock "smeared", I would have called the performance with the clock 'less smeared' and it was the fact that I heard the system not sounding as good as I know it can that made me ask if he had clock sync. On the basis Marantz would not have put an external clock input on their system (I have not actually seen a Marantz produced clock so it is hardly a "marketing gimmick" [others words not mine] for them) then clearly they think it can make an improvement. I have seen dCS U-Clocks sell for £1800, though the last one on ebay was asking about £2600. Please note I am not suggesting that you cannot get a better wordclock than the dCS U-Clock, in fact I am not commenting on its comparative capability, expect that it made an £18k dac and a £20k transport sound even better as well as making a then £11k list standalone Puccini player sound better. That I note was both in the case where the Puccini was operating though a (for the avoidance of doubt) not clocked pre-amp (that had a dac but analogue imput was used) when through a passive pre-amp (Music First Baby Reference) and when the Pucinni was directly driving a power amp directly [Amps tried - Rotel 'cheap' off ebay 981, Bryston 2.5 SST2, Vitus Audio SS103]. I dont know where you are located but a dealer in the UK has Puccini and U-Clocks for £6k I am not suggesting you sell your Marantz (though you might) but if you can find a dealer with second hand Puccini and clock why not take your marantz along try it with and without the U-Clock, and against the Puccini with an without the U-Clock - If it does something for the Marantz then dealer may be able to get you U-Clock individually, the one in the UK can certainly do that as I bought a factory refurbed one from them a few years ago (before I had Vivaldi). Incidentally the U-Clock was also good enough for use in a three box (I didn't have the clock) Scarlatti system where I did try it against the dac being the master clock - there wasn't a lot in it, but enough for me to keep it in the system. The only reason I bought a Vivaldi clock over the U-Clock when I upgraded was I also then had an Aurender W20 streamer which has a dongle to switch the clock on a Scarlatti / Vivaldi clock depending on sampling rate. I do note though that the U-Clock can be switched between 44.1k and 48k to provide a base multiple for the higher res requirements 96, 192m etc., It would be worth checking what the Marantz input requirements are. I'd just note I have seen a Scarlatti Clock on eby for £2795 again you may be able to go to the person selling and try with your Marantz if you are in UK - if not try similar in a dealer where you are and let your ears judge.
