A few months ago, I switched to using a USB DAC (HRT MusicStreamer II+) as my primary source. Fed from my MacBook Pro, sometimes the results were absolutely stunning and sometimes the results were absolutely stunningly bad. It seemed that the audio could be great one day and horrible a few hours later.I spent a couple of weeks trying to figure out what the hell was going wrong with my system. I tried every hardware remedy I could think of to no avail. It turns out that many things go on behind the scenes in your computer that can dramatically affect the final quality of your music. When the stars, I was getting good audio. If I had used the wrong combination of apps, played the wrong video, etc, the audio system could end up configured in a way that made it sound terrible. Finally, out of curiosity, one night I found people talking about a small application called BetterSound. I downloaded, I installed, I was amazed.
BitPerfect.app (the final name under which the product was released) is small companion/helper program for iTunes. BitPerfect functions like a bodyguard for your music. It make sure that the music comes out of iTunes and makes it all the way to your USB DAC without getting harassed, mugged, or lost along the way. The differences in the audio output can be tremendous. I say "can be" because the output from BitPerfect is slightly better than the best I ever heard without it yet MUCH better than the average that I heard before using it. It is, in fact, very, very good audio. Best of all, it's the same every time you listen, regardless of what other things your computer might be up to at the time. Additionally, it also provides the big (to me) benefit of being able to route your music over your USB DAC while still letting the system sounds play out of whatever output they normally use. I very much appreciate no longer hearing new mail or IM alerts being played over my music!
BitPerfect is quite easy to use. When you start it, it starts iTunes for you. The first time you use it, you have to tell BitPerfect what output to use (your USB DAC) and you can configure some options about how to send data to your DAC. After that, start it and forget it whenever you want to listen to music. You still use iTunes just like you always have for organization, track selection, etc but BitPerfect is there in the background making sure your audio always sounds its best.
BitPerfect is available in the Mac App Store and, for $4.99 is kind of a no-brainer. Absolute win.