no sound with new DAC.....possible reasons why?

Just added a different DAC to my setup and am getting no sound. Here's my setup: early 2008 IMac - Forest .75m usb out - Schiit Modi DAC - 2 ft RCA out - Qinpu A3 - to Polk RTi A1 speakers. Everything is brand new, except for the IMac. I was running a Fiio D3 DAC successfully through optical, and then a Muse DAC successfully through USB. I didn't care for either DAC and upgraded to the Schiit Modi.
I have sound output set to the DAC. I have Bitperfect installed. When I play a song, it plays about 1 second and then goes silent. Every 10 or 20 seconds after that I will hear a half second long clip.
I have tried a powered USB hub, as I heard that IMacs prior to late 2008 may need it. No luck. And oddly, the Muse DAC worked fine with The built in USB on the IMac.
Any thoughts here?
Thanks, Jason
I think it was Harry Pearson who said: "If it works the first time it's not high end".
Be sure to check your midi settings as well
Check the buffer settings in your playback software. Make the buffer setting higher and see if that helps. If not, I would contact Shiit and ask them what you are doing wrong.
See if your DAC is recognized as the audio device in your control panel or whatever Mac calls it.
I agree you should go to Applications>Utilities>Audio Midi and be sure your DAC is selected for sound output. When you disconnected your other DAC the settings may have reverted to the default settings for the computer's internal speaker.