Max usb audio output 48kHz/16-bit?

It seems to be a restriction to max 16-bit at 48kHz for audio output on the newer MacBook Pro. Does anyone know how one can get a higher quality signal through USB?

Since the optical output was dropped on the newer MacBook Pros, USB is the only choice which is fine by me but I can't seem to get the Macbook to play at higher bit and kHz.

In the Audio MIDI Setup you can choose format but there is only two options; 44100 Hz or 48 000 Hz both at 16-bit. I have an external DAC which support higher quality connected which I thought would increase the options but this seems to be a hardware restriction in the MacBook Pro.

USB in general should support much higher formats so it must be restriction on the macbook. Does anyone know any details around this?

Thanks!

/// Chris