I'm enjoying a Centrance DACmini using USB to a 2009 Mac Mini. I suggest you try the Centrance, but no matter what you choose I highly recommend an upsampler program such as the $89 Wave Editor at www.audiophile-engineering -- it uses the highly acclaimed izotope program. Upsampling to 24/96 seems to give the best results, especially if you dither first and then upsample using the gentle setting.
~Tom
~Tom