Feeding Stereo Tool from other software on your computer
If you want to feed Stereo Tool from a program that doesn't support audio processing plugins, or for example when you just want to send all the audio (eg. YouTube video's) through Stereo Tool, you need a 3rd party program to route the audio to Stereo Tool, which installs a "virtual" sound card. You can then send the audio from other software to that virtual sound card (or make it the default sound card in Windows, so all audio is automatically sent to it), and in Stereo Tool, read the input from the virtual sound card, and send the output to the real sound card.
Two programs that can be used for this are VB Cable and Virtual Audio Cable. Make sure to get a recent version of these programs, older versions often have issues with newer Windows versions, which can cause buffer underruns or overruns in Stereo Tool.