Do I really Need a Digital to Digital Converter?

After upgrading my Terminator Plus to firmware v3.7 I felt I2S was no longer better than USB. 

Before the upgrade my findings were the same as @vthokie83 "adding the Iris DDC and I2S was not a subtle upgrade, it was a very noticeable upgrade in SQ."  

I found that the power cord, USB cable and HDMI cable each had a strong influence on the quality of the sound from the Iris.  Using the clock out from the TP was helpful but not as profound as the cable.

I also used to have a W4S Music server and 2v2SE DAC.  The USB was much better than the direct I2S connection (using W4S cables for each). 

Point is I2S is not universally better than HDMI.  

anzaanimalclinic,

I have often thought that at some point, the USB might be as good as I2S as I move through future upgrades. More expensive allows for higher quality throughout the chain. The Terminator Plus has an oven controlled (OCXO) crystal which is better yet than the temperature controlled (TCXO) crystal in the Iris....and the firmware update appears to improve the USB to a point where the DDC becomes redundant.

I look forward to that day, but until then I'll remain exceedingly happy with the Iris/Pontus combo

NO!

I have an MHDT Labs Orchid DAC in my main desktop system. For years I ran it off the digital coax signal from a SPDIF converter. A couple years ago I decided to try a DDC. Ended up buying the first generation Matrix Audio X-SPDIF converter. I installed it in the system in place of the SPDIF converter (using the same coax cable), and immediately heard small but worthwhile differences in soundstaging and the focus/palpability of notes. That's the only DDC I tried, and it's a keeper.

When the usb input stopped working on my dac and I learned that the amazon seller that I purchased from was in China and would not help me with a repair, I discovered goldensound's write-up on using a DDC. I had a Bryston DDC and hooked it up to my dac with a Requisite D3r coax cable. As a result, I am not going to get my usb input fixed. Everything sounds a bit better than before, especially on live recordings of classical music. Yet, I have used the Bryston DDC and the Eitr with several other dacs and found them to add a hooded and boring quality to recordings that was not present when listening through the usb input of the dac.