Since you're using a Pi already, you may want to shorten your signal path a bit by adding a Hifiberry Digi+ board to it, so you end up with

Pi + Digi+ > SPDIF > transformer > SpeakerLink

It'll probably sound a lot better as well.

Living Roon: MS200 > Reviver > MiniDSP DDRC-22D Dirac > DSP5200 | Headroon: Allo USBridge / SBooster PSU > Prime / SBooster PSU > MrSpeakers ÆON Flow Closed | Bedroon: Bluesound Pulse 2 | Roon Server @ NUC6i5 on Debian Stretch