I noticed far more difference between Toslink and coax through a Topping E50 than any inputs on a DirectStream.
Of course you NEED to use i2s with a PS Audio transport for features. My rec is Revelation Audio Labs HDMI style i2s cable.Good luck getting one of those. Start praying.
If you want to use the second i2s input, the Matrix SPDIF-2 is good enough for me.