Bi-amping KEF iQ9's with Rotel 1070's

Hi, I hope someone can help me. I have a pair of 1070 power amps (1 new 1 from Ebay) and a 1070 Pre Amp (new). I am using a 1072 CD player and hooking them up to a pair of KEF iQ9 speakers. I hook up L and R channels of one amp to L and R speaker bass terminals and L & R channels of the other amp to L & R of the mid/tweeter terminals. When I turn on the pre amp and one of the power amps, all is fine, butt as soon as I turn on the second power amp I get a loud buzzing/popping from both speakers and have to turn it off quickly for fear of doing damage. I have a second (Audiosource) preamp that does exactly the same. I notice that the power light on the used power amp is red and the new one is blue, could it be a problem with an older model of the power amp?
Does anyone have any idea what is going on?