Loud humming when I connect my DVD player

Your interconnects may have a broken ground. Have you tried another pair? Are the DVD prayer and Preamp plugged into the same outlet? If not: One outlet may be wired incorrectly, with regard to hot and neutral. Try reversing the phase on the DVD player(flip the plug in the socket).

I assume the DVD player is connected to a TV....
Is the TV connected to a CATV system? If so you are experiencing a ground loop. Disconnect the incoming CATV coax cable from the cable box and then check for hum.
Post back results of test.

Tried all but nothing worked except disconnecting the TV from the CATV box. So either the box or the cable is the issue. Anything I can do about this?

The earth connection of the grounding block for the CATV Coax cable shield is causing the problem.There is a difference of potential, voltage, between the CATV earth ground and the main electrical grounding system of your home.

Buy one of these and install it between the incoming CATV cable coax and the cable box.

You also could try cleaning and re-terminating the ground wire on the grounding block. The grounding block should be located on the outside of your house where the cable company's coax cable connects to the outside of the house. Sometimes the block is inside a box, sometimes just exposed on the outside of the house.

Clean both ends of the ground wire and re-terminate.
The other end of the ground wire from the grounding block should connect to the main grounding system of your home. Not a separate ground rod only. A separate isolated ground rod connection will cause a ground loop condition as well as a lightning hazard for your audio and TV equipment.