The other day I wound up doing a blind A/B comparison between the Toslink and RCA S/PDIF connections on my Logitech Squeezebox Duet music server. I hadn't intended to do this, but when the opportunity presented itself, I couldn't refuse.
The Logitech is part of my "warm-up/break-in" system where I put new gear before I begin serious listening. Currently it's made up of a Logitech Squeezebox Duet connected to a NuForce DAC-100 DAC, which is connected to a pair of Adam A-5x self-powered monitors. The system is on summer vacation while I'm waiting for some new stuff to show up.
I was doing some work in my shipping/receiving/ room when I decided to turn on the system (which I hadn't listened to in several weeks) to see if I needed to do any updates on the Logitech. After doing the latest firmware update (and possibly the last since Logitech discontinued the entire Squeezebox line of products,) I decided to listen to the system. That's when I realized I had no idea which of the Duet's two digital connections was connected to the four digital inputs on the NuForce. Unlike some DACs, which label their front panels with specific names like "Toslink" and "S/PDIF," the DAC-100 just has "1", "2", "3," etc. That's when I thought to myself, "What a perfect opportunity for a blind test." So I put on some music that I know well and began listening.
After listening for over ½ hour I found that one of the two inputs was noticeably better than the other. Remember, that I had no way of knowing which of the two was Toslink or S/PDIF. When I finally looked in the back of the NuForce to see which input I had chosen, I discovered that the S/PDIF had been the one I had found to be sonically superior.
The differences were easy to hear. Through the S/PDIF every aspect of the presentation was better - slightly bigger soundstage, better definition, more inner detail, and increased dynamics.
As you probably know, Toslink consistently produces higher jitter specifications than S/PDIF, which is why standard audio "best practices" is to use S/PDIF rather than Toslink digital connections between a transport or digital source and DAC.
My little blind test proved that in the case of the Logitech Squeezebox and NuForce DAC-100 the S/PDIF connection was the best-sounding connection methodology, which is perfectly in line with best practices.
While for some readers this whole thing may produce a big yawn accompanied by "So What? Another battle already fought and won." But for me it's one more piece of evidence that indeed some things are audible, even to a blind-testing audiophile.