i2s Cable for Denafrips Iris -> Pontus?

Hi all! Finally got all the pieces to my digital front end puzzle and am getting things hooked up and set up.

What I landed with:

Innuos Zen mk3
Denafrips Iris 12th DDC
Denafrips Pontus II 12th DAC

Right now, I have USB running from the Zen to the Iris DDC, and then using an HDMI cable for i2s out from DDC into the Pontus. I got things setup to where sound is playing, but I have some observations so far.

1. I was using the Zen into a digital input on my integrated. It sounded ok, but wasn't great. I wasn't expecting much, but wanted to have a baseline to see how the Denafrips gear would improve on this.

2. Connecting the Pontus on its own to the Zen, there was an immediate, drastic improvement in pretty much every aspect of the sound.

3. Adding the Iris and getting it configured is where I ran into an interesting issue. No problems getting sound to work by following the walkthrough video, so that was good. However, there seems to be a very slight muffling to the sound overall vs the DAC alone. The sound is less holographic and it took some of the sparkle off. To me, currently, it sounds like a slight downgrade vs the DAC on its own, which shouldn't be the case.

While some of this may be 'fresh out of the box' syndrome that break-in might change a bit, I suspect that possibly using a random HDMI cable for the i2s isn't the best approach and may be contributing to this issue.

Not sure if others that have this combo have run into this at all. If so, would like to hear if the cable choice was the problem or if it was some setting somewhere. The instructions for these aren't the most thorough, so it's definitely possible it's a setting issue as well.

Thanks for any help!