First, identify the culprit component for sure. Turning off other devices with their cables connected may not be sufficient.
1. Disconnect all cables from the amp except for the speaker cables and power cable. Does the problem go away? If not, then it likely is your amplifier. How long are your speaker cables?
2. If the problem did go away, next plugin your pre-amp, but with nothing else plugged into that. Did the problem come back?
3. If the problem did not come back, plug in one source at a time and identify the one that is the culprit.
Once you have identified the culprit, then you can start taking steps to address it. It may well be the amplifier, but do the above to make sure before you spend too much time/money on it.