I suggest you to unplug your equipments one by one (e.g. sub-woofer, tv, cableTV receiver etc), and see whether the problem disappeared after removing.
Also only plug in those interconnect cables from CD player -> Preamp -> Power Amp only and unplug others to test so as to isolate the problem.
If the hum sound still exists after these process, you can try to remove the ground from the power amp to do the test again.
The following are some information for your ref
http://www.epanorama.net/documents/groundloop/
Products that reduce hum (but you need to identify the problem source)
http://www.21best.com/21_best/electronic/security/video/filters/for_sale_.html
