I agree with Tvad.
Still have that hum I see. I doubt that the way you had connected the sub caused your hum though. You will find out when you change it.
Assuming it does not I would suggest that you attack the hum issue more aggressively by unplugging and disconnecting everything except the amp and main speakers. Assuming that there is no hum with just the amp and speakers, you add back into the system each piece (one at a time) going back up the chain. After each piece is added you turn on the system for a couple of minutes. You will notice when the hum reappears. When it does try adding a 'cheater plug' on that item and/or the amp and see if it goes away. Be very methodical, and although I sure you needn't be warned, be sure you turn off the amp when making any additions or deletions - you could easily damage the amp or speakers.