I have a Cambridge DACMagic.

I auditioned a SB3 for a week or so. Like the above poster with his VDA-2, I did not find that the DACMagic provided a substantial improvement over the Squeezebox's internal DAC.

The DACMagic makes a big improvement, however, when I stream music via MacBook > Airport Express > Optical out to DACMagic > analog out to RR2150. This is no surprise, as the AE's DAC is pedestrian.

It's also a noticeable improvement over the RR2150's USB input, FWIW. Hope that helps somewhat.