I think you need to verify whether the hum is coming from the amp or from the speakers. If it is coming from the speakers you probably have a ground loop and that has nothing to do with the Mac really... that can be resolved by floating some of the grounds in your system.
If the hum is coming from the amp itself and not the speakers, that is another matter entirely and could either be due to a problem with the amp itself, in which case you should have it checked by a tech, OR it could be a problem with your AC in which case it could eventually cause damage to your amp. If your AC is either too low or too high (but more likely too low) in voltage, the transformers may be humming as a result. You should see if you can verify the voltage of the AC jack you've got the amp plugged into. If it is way out of whack your electrical company should fix it free of charge although that will depend on your geography (here in Chicago they have to provide AC within a fairly strict margin of error or they will come fix it for free) As an absolute last resort you could plug your amps into a line conditioner but I discourage this because I do not like plugging amps into such devices as they typically compress dynamics and have other undesireable side effects.
If the hum is coming from the amp itself and not the speakers, that is another matter entirely and could either be due to a problem with the amp itself, in which case you should have it checked by a tech, OR it could be a problem with your AC in which case it could eventually cause damage to your amp. If your AC is either too low or too high (but more likely too low) in voltage, the transformers may be humming as a result. You should see if you can verify the voltage of the AC jack you've got the amp plugged into. If it is way out of whack your electrical company should fix it free of charge although that will depend on your geography (here in Chicago they have to provide AC within a fairly strict margin of error or they will come fix it for free) As an absolute last resort you could plug your amps into a line conditioner but I discourage this because I do not like plugging amps into such devices as they typically compress dynamics and have other undesireable side effects.