SCHIIT is as good a place as any to start.
S/PDIF done correctly should sound better than TOSLINK but may not due to other reasons.
ALLO DigiOne should be at the top of the list, if others here are to be believed. I have had good luck with the HiFiBerry Digi+ Pro.
Next up is a REALLY good 5V supply. Spare little expense here. I tried my $400 LPS-1 UltraCap supply that normally powers my uRendu on my Pi (LPS-! has a voltage selector switch which includes 5V) and it made a world of improvement over bith the iFi iPower and the Jameco LPS, one of which I usually use to power the Pi.
No suggesting a $400 supply makes any sense for the Pi, but it does show how important clean power is when dealing with digital audio. Maybe a TeraDak 5V supply of something of that sort? Some have good luck with high capacity 5V rechargeable battery packs.
