This post made me get up and look at my pile of old crap... Yep, I have a KHS-6640 with 7 keypads and the users manual. I installed several of these about 5 years ago. From what I remember there support was pretty good, I had to change out a few amp cards and they were sent out right away, no charge and no questions. Most likely it was a grounding issue, but I remember having to put shorted rca's on any unused source inputs.

I checked, and I still have their number for support. I dialed it and it still works, email me if you want it.