Hi,
As a matter of course, you have to plug the device into the machine before you can select bit and sample rates. The computer then communicates with the attached device, after which supported bit and sample rates are shown in AudioMidi. As an example, my current USB dac only supports 16/44.1 so that's the only option available to me. Prior to this, I had a 24/96 USB dac so bit and sample rates through 24/96 were available. Tiger does support 24/96. Run Software Update to see if any newer versions of iTunes are available to you.
I've heard nothing but good things about Empirical's product line, so I don't think you can go wrong there.