Testing Goldhorn DSP Frequency Response

In this video, I'm trying to find the low end frequency response of a Goldhorn 20 channel dsp. It has a rated response of 20hz to 40Khz. I'm using a cheap tone generator on the RCA inputs to play super low frequencies. I would like to be able to test it using the digital inputs. If anyone has an idea of how I can do that, let me know. i could probably get an analog to digital converter, but I don't know if they are designed to output super low frequencies.