I suggest that
you take your electricity for the computer and amplifier
and all that get's physically connected to them from one grounded outlet.
And beware if you have antennas connected to the system, they give ground
loop humming also.
I tried putting everything on the same power strip and I still get the
noise. I have a mixer and an amplifier that the signal is going
through, and I get the same noise going:
Just because you are plugged into a three prong outlet doesn't mean it's grounded.
Sometimes when electrical outlets are wired, the ground connection isn't actually
grounded and is useless. Sometimes power strips have a little light on it to show if it is
actually grounded or not. Also, cheap power conditioners can help alot. (Rack Rider for
instance)