If your amps do not have capacitors blocking dc at the input, then the amps will amplify dc and you will hear it on the output or the amp's dc protection circuitry will shut it down. It sounds as if you have excessive dc offset on the amps. or from the pre-amp. if the amp does not have a dc offset adjustment, then it is possible that a device in the amp has or is failing causing a dc offset. you would have to open it up or take it to a qualified tech to check each component for correct operation or failure and replace/repair as necessary. If you disconnect the amp from all other equipment,plug it in and turn it on and the dc offset is still there, then it is the amp and must be repaired. amplifiers that are well designed will amplify any signal, be it ac or dc. dc at the input will cause big problems if the amp doesn't have dc protection circuitry. Such as blown amps or speakers. or if it does have such protection, it will simply shut the amp down until you find and fix the dc offset problem. If your power supply filtering isn't up to snuff, the dc on the ac line may find its way to the audio amplification path. So, if this is the case, you may want to check the power supply capacitors and bypass capacitors to see if they are leaking, going bad or failed. Or add some bypass capacitors if they aren't any there. DC on the ac line shouldn't be there. But if you can't find the source, then an ac isolation transformer or a power line conditioner is necessary.
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