From the website:
Class Compliant Drivers
As a class-compliant audio device, the USBPre 2 is limited to a
maximum data rate of 24-bit, 48 kHz in Windows, Linux, and Mac OS
versions 10.4 to 10.5.7.
Mac OS 10.5.8 and higher support the audio class 2.0 device and can
address the USBPre 2 up to 24-bit, 192 kHz.