Hey,
Personaly I would highly discurage from conecting a AMP output to a line in input from a active speaker.
The Settings Selektion is a bit missleading.
On a nother board with funktional AMP and Headphones output, you could deside betwen:
Heaadphones -> AMP goes out when something is Pluged in to the 3.5mm Jack
Subwoofer -> Both AMP and 3.5 stay o so that you could ad an Active subwoofer. but it dose not Change the output. the Subwoofer needs to filter itself.
For active speakers I recomend you use other boards with headphiones output or you wait for Raspiaudio to bring the one out they mentioned on 408.
Hope it helped