@soix Before you connected the Iris your DAC’s USB input and card most likely took over processing duties from your Zen.
However that is not possible with the Iris connected.
As i said earlier the Iris is not a clock, it’s a re-clocker, so it re-clocks the incoming USB signal from the Zen and passes that cleaner signal to the DAC.
I2s will generally sound better because it’s far less prone to jitter than other digital connections. RJ45 will probably sound better than HDMI.
Please remember this is USB being converting to I2s so although it’ll sound good it’s never going to sound as good as native I2s direct from a source component, streamer, CD/SACD etc.