There are a number of ways that the output from a headphone amp work:
1. Some are just line out pass through so one can put the headphone amp in the chain if one's amp/pre-amp doesn't have a tape out. In this case the volume control on the Woo will not impact sound. This function allows one to stick a headphone amp between the source and speaker amp without impacting speaker amp SQ.
2. Some are passive pre-amp (MAD are like this) where the signal is attenuated by the Woo's volume control but their is no added pre-amplification. You hear exactly what the source produces, but you can adjust volume. This function is ideal for powered speakers.
3. A true pre-amp will amplify the signal from the source and send a stronger signal to the amp than if the source is attached to the amp directly. Decware CSP driving the Taboo is an example of a true pre-amp that impacts the SQ rather than just allowing volume control function.