Dear Berwhale -- How high did you push the bit rates over your optical cable?I ran my D50s over optical for many months to break a ground loop between my PC and the DAC. It worked perfectly.
I switched from digital coax to optical between my BlueSound Node 2i and my D50s, and it seemed to work fine for a while, but more recently I'm occasionally having what sounds like high-frequency "static" -- not the same as the earlier "crackling" -- over the music, apparently only at sampling frequencies above 88.1 kHz. (This is Qobuz limited by the Node 2i to 96 kHz. All I really know is that the D50s reports 96 kHz and the Node 2i says "HR" when this usually happens. So far I haven't diagnosed further.)
Actually the noise sounds more like some kind of RFI interference than "buffer underflow." No actual dropouts so far. -- JClarkW