If the preamp's power cord has 3 prongs, try a cheater plug.
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Can't say this will work, but it did for my amp's transformer hum. Try to find which circuit the pre-amp in question is running off of. Then check the circuit breakers in the service panel. If per chance, your pre-amp's circuit is on the opposing phase of some of your other audio circuits then it is possibe that some AC noise is being generated thats affecting the transformer. If so, then have an electrician move all of your audio circuit breakers in the service panel to the same phase of 110 volts. That cured my amp's transformer hum.
Keep in mind that I've had 5 used amps on this same circuit without any hum. My last amp hummed until I moved the amp's circuit over to the same phase as the others.