what model pioneer?
rather than a variac, it's usually better to use a "lamp tester". you need a power cord, lamp socket, switch and outlet. the neutral side goes straight through from the power cord to the socket. the hot side goes power cord>switch>lamp socket>outlet. for a 100W/ch amp, you screw a 100W lamp in the socket. for a larger amp, use a 150 or 200W bulb. plug in the amp to the outlet and replace any blown fuses. what will happen when you turn the switch on, and turn on the amp power, if nothing is shorted, the bulb will light briefly and go dim. if something is shorted, the bulb will grow brightly, and the line voltage to the amp will be minimal.