Cables and Loud Hum

I've got a Vandersteen subwoofer in my system -- still using the adjustable x-over box. I'm replacing my cables with Nordost Blue Heaven cables and got my 1st pair last week and hooked them into my system last night -- from the x-over box to the amp (VTL MB 185). Left the Monster cable connecting the pre-amp (VTL 2.5) to the x-over box in play. Powered up and before I started playing any music there was a loud hum coming from the system. I immediately powered down and removed the Nordost. I'm guessing it was stupid for me to assume that I could run the pre-amp to amp connection in this set up with two different cables, but here is my question: What is the technical explanation for the problem I expereinced? I've got another Blue Heaven coming, which presumably will solve the problem. Thanks in advance for sharing your thoughts/expertise.