Hopefully this will help.
The pre-amp out from your sound card will be an analog signal so no sense in plugging it into a DAC. A DAC needs a digital signal.
You should try to find out if the pre-amp out needs to be selected in the audio software otherwise it will not receive a signal and no amping will be possible.
As long as you are using a stereo to RCA Y-Splitter from the pre-amp out you should receive a signal to your amp as long as there is signal going to the pre-amp out on the soundcard. This is the first hurdle to overcome.
I am not sure if there is an input selector on the Woo, but because there are multiple inputs you should check the manual to see if you need to tell the amp which inputs to use.
Silly question, but is the Woo powered up as in the tubes warming up and glowing?
As far as the DAC question connecting it depends on what DAC you buy. If it is say a USB DAC you connect the DAC to your PC via USB cable. It will most likely install some drivers. Once this is done you should choose it as your audio source in the audio settings so the DAC receives the digital signal not something else.
With the DAC now receiving the digital end of things use RCA interconnects to hook up the DAC's output to the Amp's input. The DAC outputs an analog signal which is not only what the amp needs, but what headphones need too.
All of this should be done with the amp off and with the volume at 0%. Always turn on / off an amp with the volume set to 0%. Plug in your favorite set of headphones into the headphone jack. Turn everything on and play some music on your PC. Set the output volume on the PC to 100% and use the amps volume knob to control the volume when listening to headphones. If the DAC has a volume knob turn it up to 75%.
At this point you should hear music, but depending upon what player you are using you may need to select the new DAC in the preferences. This is the case with Foobar2000 for example. Some USB DAC units like USB 2.0 ports better than the more modern USB 3.0 ports. Try that solution if there appears to be a problem you cannot solve.
I hope you get your music up and running.