Just because there's been more than one voltage said to be optimal for the Opus power supply, let me quote directly from the Opus documentation.
"The Opus is designed to utilize separate analog and digital power supplies from 6.5V to 15V. They do not need to be regulated, but a good dual regulated supply like the LCPS is recommended. There are two on-board voltage regulators to provide power to the DAC. If you use the LCPS adjust both output voltages to 7.5
for good results. This will provide more than enough voltage drop for the on-board LDO VREGs while not making them dissipate more power than required."
Just a note about pre-regulators, which is what you guys are planning on using the Sigma 11 for. These things need not be so elaborate to get the DAC to operate well. Depending on what sound you're looking for, an even simpler regulator could give you better results. A less regulated supply using a simple pass transistor without any feedback could give you looser, fuller bass and more overall warmth. I tend to like these more for pre-regulator use, but that's just me.
On that note, I do like the circuit topology of the Sigma series regulators, which feature very good stability into various loads. But man, for the current that the Opus or two Opuses are going to be drawing, TOTALLY overkill
But heck, going overkill is one of the advantages of DIY. Have fun guys.