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.
buy a jitter device hat goes between the mac and krell. i use them in every system i have. also, the krell might not have a quality dac inside it. before going to a streamer or vdac, and since you have a krell, i would suggest a good tube based dac like the music hall 25.2, or a older cal audio.
Are these tunes downloaded from Itunes?
I imported the music files to iTunes from my own CDs. Thanks.
thanks for the suggestion, Rbstehno.
Number one thing (IMHO) make sure you never use a plastic TOS. Use a real-glass TOS if you are not already doing so. It's a MASSIVE change for the better.
What's the bitrate? 128k 192K, 256k, 320K or lossless?
lossless AIFF files
try aiff or apple lossless. i agree with using glass over plastic for toslink.