I am at the moment running in a HDMI cable (Harmonic Technology) on my Singxer to the PS Audio direct stream. It has approx. 20 hours on it. Does it sound better than a cheap HDMI cable, easily yes. On some recordings eg any Antonio Forcione the sound quality is extremely good, better than Spdif in detail, soundstage and depth. On other less recordings the sound at the moment is a bit lean and the spdif is more forgiving. The sound using I2s is clean, using spdif I can hear some distortions in the background. The HDMI cable is supposed to lose its lean sound in time but at the moment it's good to have both connections
I have read that the AES connection can be better than Spdif and I2s. If I can get a loan of a AES cable I'll report.