Windows SHOULD work this way, but it doesn't. You're best off setting the volume to full up on Windows, and just leaving it that way and adjusting from the audio processor. All it is doing is modifying the signal to pad it down instead of passing it as-is. This is true of ANY computer digital out.
P.S. Set your iTunes volume control to full-up too.
You dont' always have easy access to whatever is receiving the signal digitally. it would be nice to have the option to adjust it.
Example, I have som Roland DS-50A studio monitors, the volume controls are a knob on the back. I have to reach around behind EACH speaker to adjust the volume, independantly. This sucks.
So I just run analog so I can control volume in OS X.
OS X should work like windows on this one, not the other way around.
Volume doesn't really make as much sense for optical, in since it's transmitting raw audio data. Generally you adjust the audio at whatever the last digital source is before the receivers.
It's kind of like changing the volume of an MP3 file. You can do it, but you're going to have to alter the file and likely degrade the audio. Same reason devices you plug into your TV don't have their own volume controls. You'll notice the XBox 360 doesn't have it's own volume control.
Or, the last digital receiver doesn't have convenient volume control (like the knobs are on the back of the speakers...and in my case each speaker has it's own knob you'd have to reach around behind AND adjust each one independantly. . . that sucks too)