SOme info on the web regarding headphone mixes in the Audiobox 1818VSL reference settings in the little mixer program that was also their interface settings app, and Presonus did away with that mixing menu app some time ago.
 
Just be aware of that when looking at any retrieved info on using headphones with that interface.  I happen to have my interface output feeding into a mixer and THEN to direct boxes and speakers, so I always just use the headphone jack on my mixer to monitor everything there - meaning no direct hardware monitoring, HOWEVER I AM getting to hear it as it exists in Sonar that way, and for ME, that works just great, as my latency when tracking is running around 5 ms each way using 128 samples for my ASIO Buffer Size.  (When mixing, I always bump that buffer size up to 2046, since latency is not an issue when playing back during mixing or mastering).
 
Bob Bone