Acceptable speaker hum from integrated or sign of impending failure?

I have two integrated amps that are now about a decade old and one produces a slight 60 Hz hum that is audible < 6 inches from speakers when power is on and no inputs are connected- the other is dead silent. The hum doesn't increase in volume as the knob is turned and is constant. I've moved the "hummer" to different circuits and no matter where it is plugged in or what is plugged into the inputs or outputs the hum is audible. Also tried swapping the three prong cords between the amps and this has no effect. Only had it a week or so and am not sure whether the hum is new or was unnoticed previously. Is this an acceptable shortcoming? My main concern is whether the power supply or any internals may be going bad and causing the hum.  Any thoughts greatly appreciated!