Connect your source (CD or TT) to pre-amp, connect your pre-amp to the amp and connect the amp to the speakers. Hum? yes? then turn off components and disconnect your source (CD or TT) from the pre-amp. Turn on components. hum? no? then it is the source equipment or the interconnect cables from the source to the pre-amp or the power connection of the source. Hum? yes, then turn off components and disconnect the pre-amp from the amp. turn on amp. Hum? no? then it is the pre-amp or cables from pre-amp to amp or the power cable connection of the pre-amp.
Always start with everything connected and hum present, then work from the source-pre-amp-amp in that direction to eliminate the problem. Remember, it might well be the interconnect cables. Some connect signal ground to the shield and may cause ground loops also.
Under no circumstance do you ever use a cheater plug. I can't tell you how many times I see people suggesting the use of cheater plugs that have no idea that they are actually suggesting that you violate a basic safety issue and may cause serious injury to you, your family, pets, etc. Do not ever use cheater plug. Find the problem and fix or remove it instead. After you have taken the steps listed previously to find the faulty piece of equipment or connection, connect all of your low level signal sources (CD, TT, Pre-amp, Tuner) into the same power conditioner or outlet block which then plugs into its own wall outlet directly to the panel. Then plug your amps into their own separate outlets that go directly to the panel. So, at minimum, two totally separate outlets wired straight to the electric panel. If you have more than one amp (bi-amp) then separate outlets for each amp wired to the electric panels. In this case, three dedicated outlets wired individually to the electric panel. Noise floor significantly drops and no ground loops.
No cheater plugs. This is like someone telling you to drive with no hands. Sooner or later you will run into something.