good troubleshooting procedures so far... now,
5. Check the cable that comes from the dock into the back of the sub, pull it out an check for bent pins, and then plug it back in.
6. Remove the grills, with no music playing put your ear up to the speaker, right in the speaker, and listen for hiss... compare the amount of hiss from the left and right channel. You should have abou the same amount of hiss in both channels...
7. If you have the same amount of hiss it's time to check the source and the cable going to the music source, or try a difference source
8. If there's no hiss in the right channel but hiss in the left, try one more time and switch the speakers and wires. Even if the source or DIN cable is bad you should get a little bit of hiss in both channels.
9. If the DIN cable and source check out OK it time to move to the dock. If you have a cable which you can plug into the line input on the back of the sub and the headphone jack of an iPod that would be good. This input bypasses the dock completely. If this doesn't work (assuming your souce is good) then the amplifier channel is bad. If it works out, then you've got either a bad dock or a bad connection between dock and sub.
10. next try the AUX input on the dock... this bypasses the iPod dock connection if this works out you've either got a bad connection on the dock or on the iPod, and you should try a different iPod dock to make sure the iPod is OK.