Should I add a DDC?

Another thread here addressing the same. I won't go into detail as I did on other thread, so in a word, yes, and get the OXCO clock Hermes or Gaia.

Number of issues on purchasing ddc for I2S. Very few streamers offer I2S output so in these cases ddc required for using I2S. Another concern is quality of clocks in ddc vs dac. If dac internal clock is equal to or better than ddc clock you're not getting any benefit from clocking with ddc, only possible benefit is the I2S pathway. Greatest benefit with ddc is when it's clock superior to dac internal clock, in this case more likely to be superior to usb. Whether it superior or not depends greatly on the quality of that clock, a high quality OXCO clock with high quality power supply to it is optimal. How these clocks implemented critical, not all clocks created equal, such that a poorly implemented OXCO clock could be inferior to quality implemented TXCO or Femto clock.

And then we have concern of input to ddc, generally this going to be usb, this should also be optimized for highest performance. A noisy or poor quality input to ddc will greatly impact ultimate performance.

My point in the above is don't automatically expect I2S to exceed usb, or in some rare cases other interfaces. Many dacs have sophisticated usb boards, less than optimized I2S will likely be inferior, especially if you have usb chain already optimized. A previous foray into I2S using Singxer SU6 and generic I2S cable proved inferior to my optimized usb setup.

I2S cables should be kept as short as possible, .5M or shorter best. I2S was never designed to transmit via cable such that theoretical best position for clock is just prior to I2S path in dac. Ed Meitner of EMM Labs explains this issue for those interested in the whys and wherefores.

One should also purchase an I2S cable vs HDMI cable, you only need 8 pins wired to transmit data and clock. And if purchasing more expensive silver cable this can be money saving, silver values being what they are. I've also found quality of cable can make a meaningful difference as well. I've only had two thus far, but the Tubulus Ximius easily bettered a $70 copper cable I was using. Both Audiolund and Tubulus make more affordable cables than my Ximius, but may still be above one's present budget.

As for extra cost of ddc and cable vs using that money for a better dac, one can only speculate. I could make a strong case the ddc won't become obsolete as long as one continues using dacs with I2S inputs, it brings out full potential of these dacs whereas usb this not likely. My Musetec 006 has pretty much state of art usb board and I2S easily bests it.

Since the addition of Denafrips Gaia, Musetec MH-DA006 dac, Tubulus Ximius I2S and new power cables have been recent additions I've been waiting for system to stabilize (over 300 hours burn in) and for me to acclimate to particular sound qualities of all this new equipment. So, in evaluations of syncing to external clock (Gaia) vs using internal dac clock it has become patently clear a limitation of this dac, and perhaps most if not all dacs is the I2S internal clock. When you look at the OXCO clock in Gaia and Hermes you'll find a rather intensive power supply for that OXCO clock, even if a dac implemented an OXCO clock doubtful they'd find enough real estate for it. On top of that, most dacs devote a fair amount of space to usb boards, space I'd think better used for I2S clock power supply.

The implications for sound quality are easily hear using Gaia clock vs Musetec clock in that using Gaia clock sound staging greatly enlarged and images far more dimensional, greater density, Sense of reach out and touch performers vs the relatively flat presentation of internal clock.

Beyond this, in my evaluations of optimized usb vs optimized I2S, I2S easily surpassed usb, and maintains this superiority even when bypassing Gaia clock.  From this I surmise I2S alone (without using ddc clock) is superior to usb, this points to inherent I2S superiority via it's dedicated data and clock runs, and the elimination of detour through a usb board (I2S is native signal path in dacs). the better clock in Gaia is added major benefit. Based on this reasoning I'll extrapolate a quality implemented OXCO clock in a ddc using I2S interface should outperform any dac's usb implementation and/or likely inferior I2S internal clock.