Should I add a DDC?

The value of a DDC is much more than just converting to I2S. It does several things depending on the product, since I have a Denafrips DDC I'll speak there.

Convert the input signal to I2S (used internally in DACs/streamers/etc), galvanically and optically isolates the signal to eliminate noise and jitter, re-clocks the signal using the DDCs crystal oscillator.

Prior to the 15th versions, the Denafrips Ares and Pontus DACs use high quality FEMTO clocks, the Venus used a TXCO (temperature compensated crystal oscillator), and the Terminator II and Terminator Plus used OCXO (oven controlled crystal oscillators).

After the 15th, the Ares and Pontus use a LPNO (Low Phase Noise Oscillator), the Venus uses OCXO, and the Terminator 15th uses dual OCXOs, and I have no idea what a Terminator Plus 15th would be when it is released (if it is needed)

SNS

As you pointed out, you do need to be careful when adding a DDC to a DAC. You never want to downgrade the clock, so you want the clock in the DDC to be as good or better than the clock in the DAC.....in almost all cases, the clock in the DDC takes preference over the clock in the DAC

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.

Thanks All, very helpful! Thank you again @jastralfu I was going to ask where can I get the test tones? I'd have to play them through my streamer, correct? I will have Eversolo to Hermes via USB then HDMI from Hermes to Pontus. Where can I get the test tones to play through my streamer/Qobuz?

I don’t know about streaming the test tones.  The link I posted earlier will download a zip file so likely you will need a computer hooked up to your ddc to play them.  If you have a Mac then it’s easy just connect your Mac via USB.  If you have a windows machine you’ll need the USB driver for the Denafrips but you can get that from the Denafrips website.  This link I believe is what you need for the windows drivers

https://www.denafrips.com/download-thesycon

One caveat that I am just learning about is i2s incompatibility. Make sure that your DDC's i2s is compatible with the DAC's i2s when you have different brands. One good thing about the Hermes is the ability to adjust the hdmi setting.