Preamp/tuner/headphone question

I purchased a B & K 5-S2 preamp/tuner while my even older PSE preamp and tuner are having a bit of service performed. The B & K works fine but I have one question regarding using my headphones. When I select headphone mode my speakers continue to operate along with the headphones. I'm using a pair of PSE monoamps into a pair of Vandersteen 2Ci's. The only way that I can use headphones only is turning off the amps. I believe the amps are supposed to be bypassed when the headphones are plugged in and mode is showing headphones on. Any suggestions or?
I have the same problem with my B&K Reference 20. What a waste of a headphone jack. The manual sortof explained what to do but it was so complicated that I messed up some other settings so I gave up. check out the manual. It might say how to.