I have just purchased a Line 6 Spider IV 75w Amp.  The manual says that the head phone jack is also used to connect to a PA system.  When you plug into a PA does the Amp volumn shut off?  If so can this be over riden?  If so would the Amp volumn be controled independent of the PA system so both could be adjusted independently?  I would like to use my Amp as a monitor and be able to adjust my volumn and at the same time allow the PA system to be adjusted on its own.  Any ideas?

Thank you,

Harley-Doug