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.
I had a pair of Monster interconnects which weren't soldered correctly and caused a ground hum in my system.. removing them solved the problem.. they went into the trash. (I spent way to much time troubleshooting the problem)
Sometimes it is due to the Outlet also. Chanfge outlets or any other thing you can think of.
The Nordost Blue Heaven has almost no shielding which makes it very sensitive to any interference. I use the the Blue Heaven and Red Dawn in my audio system, but found that the system had to be well away from any tv, antenna wire, etc. I called Nordost at the time and they said to keep the cables away from such sources of interference and also listed external speaker crossovers as an interference source. A local Vandersteen and Nordost dealer has told me the same thing. When I moved the interfence sources away from the audio gear, the problem went away. I also once tried using Red Dawns for interconnects between a turntable and phono stage. That didn't work either and resulted in the same type of hum.
Well using unsheilded wires are better to use ,but there are those times when interference creates problems.

Sometimes hum can be due to the Transformers in the amps we use.

If you can get away with not using sheilding it is always better for home audio.

I addressed interference in my FAQ page.