Andrew, If your preamp is in standby then its not the preamp.
The focus should be the amplifier. You mentioned that your amplifier is dual mono.
It is possible that the slight hum you hear is due to the output transformer exhibiting vibrations from your AC source. That would explain the reason why you are only hearing it out of one channel and not the other. Its not a big deal at all from the way you described it, but if you feel like playing detective here is what you can do.
listen during different hours over the next week or so. See if the hum goes away during late hours. If it does then its the AC coming into the outlet. You can also try plugging into a line conditioner or a grounded outlet.
Either way I would not classify this as a problem.
The focus should be the amplifier. You mentioned that your amplifier is dual mono.
It is possible that the slight hum you hear is due to the output transformer exhibiting vibrations from your AC source. That would explain the reason why you are only hearing it out of one channel and not the other. Its not a big deal at all from the way you described it, but if you feel like playing detective here is what you can do.
listen during different hours over the next week or so. See if the hum goes away during late hours. If it does then its the AC coming into the outlet. You can also try plugging into a line conditioner or a grounded outlet.
Either way I would not classify this as a problem.