I'm no expert but i think the problems might be caused by the use of unbalanced cables. I don't know if it will account for all of the situations previously stated but the tutor on my sound engineering course said that external interference can be picked up if using unbalanced cables. I wont bore anyone with the details but unbalanced cables only have two conductors the conductor that carries the "hot signal" and one is the ground. Balanced cables use three conductors, one being positive, one being negative and the last the ground. Basically the balanced cables eliminate external interference (radio frequencies etc.) by using phase theory (blah blah blah boring theory stuff...
). My suggestion would be buy balanced cables although I know this isn't the only solution. I know most if not all recording studios don't use unbalanced cables due to the possible interference. Can't think of any reasons speakers would be the problems.
Hope it helps! Feel free to correct me if I'm wrong.