humming in sub

hi ive fitted a sub with a built in amp not long ago it was working all good no noise or humming, and when i was on my way to work the other morning i could hear the sub wasnt working so i pulled over in a garage opened the boot and saw that the power lead had come off the terminal so i went to put it back on and before i realised it was to late it sparked i left the engine running (like a twat)and ever since it makes a humming noise.i thought i had blown the earth in the amp so i bought a new one connected it up the same way as before but with the engine off this time but it is still humming when engine is running but works fine with just the ignition on

any ideas anyone i did think headunit may of blown something

many thanks