Ground Loop Hum

I own a McIntosh MC240. Recently I needed to have the original power cord replaced and figured why not replace it with a 3 prong grounded cord while I'm at it. Only problem was when I hooked it up to my pre amp [MFA Magus] I got hummmmm. I've been getting around it by using a 3 prong to 2 prong adapter on my pre amp. That stopped the hum, but I've always felt uneasy about not having the unit grounded.
My question is, is there a simple, cost effective way to get rid of ground loop hum while still keeping my amp & pre amp grounded?
If not, which unit is better off grounded, or does it make a difference?
Thanks in advance for any recommendations.
Does your cable TV connection come in contact in any way with your audio system? Most hum issues you describe come from the difference in grounds between the CATV system and the house ground your audio system is using. Try disconnecting your cable TV from your audio system and see if that solves your problem.
If the CATV turns out to be the problem, try a Rat Shack ground isolation transformer on the cable line, or as a temporary fix, two of the 75 -300 ohm transformers, installed back to back (CATV>>75 ohm>>300 oh>>75 ohm)
You do not want to lift the AC ground on your preamp. All of your sources are grounded to it.

Lifting the ground on the amplifier is perfectly fine. That is why the OEM power cord that came with your amplifier is only 2 prong to begin with.

If you want to upgrade the power cord to the amplifier, then disconnect the ground wire in the male AC plug.