Also, look into purchasing an EbTech Hum Eliminator, they cost $59 US.
It's a very simple box, 2 in/2 out (1/4", balanced or unbalanced).
I had hiss/hum (60 HZ groundloop I think) in my system (P4 2.4 GHZ, M-Audio 2496, Behringer UB 1204, Behringer 2031A Monitors) and thought it might have been the Behringer gear.
Then I purchased balanced cables to conncet the whole mess together and still had hum, so I think my problems was along the lines of what Jasedee just described.
Once I bought the EbTech, I connected
two 1/4" patch cables (1 ft. cables used to connect guitar FX boxes) to my
Control Room Outs (I don't use the main outs on my DAW mixer) and into the
IN ports of the EbTech. Then I connected the
OUT ports of my EbTech to my
1/4" TRS cables that go to my 2031As.
My signal chain is dead silent now and I can crank my system and hear no hum.
This device is about $200 cheaper than anything using Jensen transformers and works damn well.
Good luck!
-Alex