USB DAC better than optical?

I am currently using my Mac Pro iTune AIF music files for casual listening by connecting the computer to Krell HTS 7.1 processor using an optical cable. Sounds pretty good but far from a good CD player. I have read about the budget USB DACs such as the Streamer or V-DAC but never used it. Will these budget USB DACs outperform the Krell's optical connection? Is the V-DAC better than the Streamer, Streamer+, or streamer II? Thanks.