Can I ground my preamp?


I have monoblocs each w/their own grounded cord and a preamp w/o a grounded cord. The problem is that I'm getting a slight ground hum from each amp. The amp manual says if this happens to use a cheater plug on the amps and to ground "at least one other component." Is it possible to ground my preamp, and if so how? Many thanks for any advice.
bojack
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Quick update here: I lifted the ground on each amp via cheater plugs and voila...hum gone. Thanks for all the advice, and I've learned quite a bit.