No problems there, Alex, there are several good DACs on the market.
A couple of points, though. The DacMagic does accept both toslink optical spdif and USB from the MacBook Pro. I've had the opportunity to try a Duet, and that's another excellent option.
More importantly - as to volume control.
It's QuickTime which controls playback on a Mac, iTunes is a music organizer with a control interface, but it is QT which does the actual playing.
In earlier implementations of QuickTime, you had to keep the volume at maximum in order to avoid degradation of the signal. If you reduced the volume, then you would be processing the signal inside the computer.
Starting with Quicktime 7, this problem was solved. As Benchmark (another manufacturer of an excellent DAC) say:
The volume controls in iTunes versions 7.X and 8.X will not cause
significant distortion. The user should not hesitate to use the volume
control in iTunes v7.x and 8.X
http://extra.benchmarkmedia.com/wiki/index.php/ITunes-QuickTime_for_Mac_-_Setup_Guide
I control the volume using my iPod Touch or iPhone. For background listening, I even use volume equalization of random playback, to avoid the volume jumping up and down between different recordings (set in iTunes preferences).
For in-depth listening when I'm in the sweetspot, I disable that function, of course.
A tip when it comes to the on-screen volume bar on your Mac, you can set that in smaller steps:
Press Shift+Option and you will find that each white square in the volume bar gets subdivisions, allowing for a more precise setting, particularly valuable with the BL3s connected. You don't have that option from the iPod Touch/iPhone, though.