I've been having an on-going issue with hum from my 505. I was told it was a ground loop issue and I ignored it, as I was moving from an apartment into my new house. Well, I moved into my house and it is still there. The humming is noticeable if I put my ear to the sub when a dvd is playing, however, when I stop playback the hum becomes extremely noticeable and I have to turn the sub off to stop it. What can I do to fix this??