Why convert to SPDIF? If you already have it in a format where you can trust the impedance, then stay there, or swap to i2s.
Bryan
If your USB is also in your DAC you would think converting is not worth the effort, but in many older DACs with a USB connector, the USB implementation was not the best. Some of the newer USB-SPDIF converters are really much much better, that even though you are adding an additional step, the sound ends up being better than if you just went USB straight to the DAC on an older unit.
I have a a W4S DAC1 that, a few months ago, had to be sent back for warranty repair on a bad RCA jack. While it was there I asked them to upgrade the USB module to 24/192, they told me not to bother and buy their µlink USB to SPDIF converter, it is cheaper, sounds better, has more features, and has better hardware support. I did, and it does sound better than using the USB on the DAC, and that DAC is only a year and a half old.