A couple of years ago, I added a Denafrips Gaia to a Terminator DAC and switched from AES3 to I2S. To start out, I used an $12 HDMI cable from Amazon. This provided a very noticeable improvement over the AES3 connection.
I later upgraded to a Tubulus Argentus i2s cable ($549) based on favorable reviews. This was an improvement, but not as significant as switching from AES to i2S in the first place.