2 digital designs cm600 amps problem

  Last night I installed 2 Digital Designs CM600 amps running 2 Memphis M5 subwoofers, 1 amp per sub, running off a newer Kenwood nav unit (didnt get the model numer). Upon testing, I ran into a problem where any time if I inserted my rca into one amp the corresponding sub would play fine, but as soon as I hooked an rca to the other amp both would enter into protect mode. I spent hours troubleshooting, but could not find a solution. Originally I had the rca coming off the head unit and y's off both amps in the rear, then I tried using the built in pass thru on the amps, then tried y-ing at the head unit and running a seperate rca to the back. I also thought it possibly might be the remote leads, having two lines run off the remote turn on line, so I tried daisy chaining them at the amps, and then a relay. Nothing provided a solution, so I tried swapping to another amp of the same model, and still ended up with the same result. It didnt matter which amp I plugged the rca into, as soon as the other one was hooked up both would enter protect mode. I tried hooking the rcas up with the speaker lines undone, which both amps turned on normally, but when I hooked the speakers up theyd enter protect mode again. I checked to make sure the subs were wired up correctly inside the box, and both were. I ended up eventually running both subs off of one amp, and pulling the fuse for the second until a later date. Luckily the customer left satisfied and appreciative of my efforts, but I am disappointed in myself at not being able to figure it out and give the customer exactly what he wanted.

  Does anyone have any suggestions of what may have caused this? Id like to know for future reference and also so when the customer does return I am prepared with a solution. Any input is greatly appreciated.

Forgot to add the install was performed in a 2007 Chevy Tahoe.