Assessing the level of hum or buzz when the amp inputs are disconnected from the preamp will only be meaningful if it is done with shorting plugs (or something equivalent) connected to the amp inputs. Without shorting plugs the amp may simply be picking up interference out of the air and confusing the results of your experiments. The other amp may have behaved differently simply because of differences in input impedance or other parameters.
As for the original question, the amount of power going into the speakers that would result in the sound level you are describing is miniscule, and will not hurt anything either in the short term or the long term.
Regards,
-- Al
As for the original question, the amount of power going into the speakers that would result in the sound level you are describing is miniscule, and will not hurt anything either in the short term or the long term.
Regards,
-- Al