HISS from Numark DM1200 mixer. Not present in speakers (self powered) when mixer off. Using XLR cables with telescoping ground lifts on each line. Monster 3500 power filter. Ideas?

All interconnected equipment MUST be powered from the same receptacle or source. The XLR to the speakers should NOT have ground lift. The power for the mixer should come from where the speakers are and be routed alongside the XLR cables. All the other connected items should also be powered by the same source. The ground bounce within a building cannot only cause noise, but can rupture the input circuits of equipment. As an electrical engineer I don't know why manuals supplied with the equipment doesn't go into this more. The professionals at the big venues all know this as they have had to pay to get equipment repaired.

Now to get down to "hiss" once you get the other stuff straightened out, there are two things to know at the start: The first is that due to the statistical nature in electronics "noise and hiss" are among the laws of physics that we can't completely get around. The level of the hiss should NOT be objectionable however. The second thing is that proper level setup is important to minimize the effects of noise and hiss. You should be sending the levels to the powered speakers at near full line level into the line inouts, never mic inputs that some have. Using excess gain at the speakers will amplify any noise pickup on your audio lines. Next, at the mixer, try to keep all the level controls around 10 to 2 0'clock for rotaries and 70% for faders. Excess gain at certain stages will cause hiss to be objectionable.