Infinity IRS Betas Make Loud Noise When On

I just recently purchased some used Infinity IRS Betas. With both base towers hooked up to the crossover, the system makes a loud noise when I turn the control unit on. If I unhook the left speaker, then I don't get the noise and the sound is fine, but I hear a low hum. Same thing if I unhook the right speaker. I only hear this loud noise when both speakers are connected. I have checked, double-checked, and triple-checked my connections, and all looks correct according to the manual. Also when I disconnect the servo control unit and play them as full range speakers, they play fine. Does anybody have any idea what could be the cause of the problem?