The Crack mutes the headphone output with no headphones plugged in. You'll need to plug in a 1/4" to 1/8" headphone adapter to get output from what you've wired in.
I tried the 1/4" to 1/8" headphone adapter method long time ago as a pre-out to my Dynaudio, not sounding very attractive...LOL
I believe DarkVoice 336 has a separate pre-out circuit from its headphone out, and uses different output cap values. There could be some differences in the output caps, not sure exactly how much capacitance is required for pre-out and/or other adjustments