I have a Monster 6 pair Speaker Selector to switch 5 Speakercraft AIM7 DT Three ceiling speakers. The problem I have is that when I switch on one specific speaker the volume of the others decreases by 50%. It doesn't matter how many other speakers are "on" it still happens. The speaker causing the problem works fine with or without the other speakers. I have tried changing the connections around but it still happens. Any suggestions would be appreciated?

At a guess you might check the impedance selector switch on the speaker; it may be set to 16 Ohms.