I'm going to presume that you already have your speakers and are happy with the Cyrus/speaker combo, so will only address the logistics of what you're trying to do - the SM6 is a wonderfully rounded source that doesn't exhibit any brittle tendencies in my experience.
The short answer is you can do what you ask but you're going about it the wrong way: the pre-outs of the Cyrus have no part to play. What you should do is plug the Cambrdge into the AV input of the Cyrus and follow the instructions in the manual to set the AV channel to have fixed gain. This will mean that the amp will always have fixed volume and you can use the volume control of the StreamMagic to control system volume (after you've set it up to run in pre-amp mode, obviously).
However, I'd point out that you're still using the preamp of the Cyrus, and if you set the fixed volume of the Cyrus too high, you could run into noise problems. I'd probably not set the Cyrus above 12 o'clock before fixing the volume. Alternatively you could just plug the SM straight into any of the Cyrus' inputs and use the amp's volume control...
EDIT - just read your note again and I wonder if you;re under the impression that it can *only* work in preamp mode? This is not the case - the default is to have a fixed output like any other normal CD player or source, and yu have to specifically set it to be in pre-amp mode. Hope that's clear.