Hello Tarjin, please feel free to e-mail us with questions. I don't look in here often enough due to work and travel to be consistent in reply.
The primary cause of hum related to the use of dedicated lines is a difference in potential to ground caused by running differing lengths of wire from panel to outlet. If your furthest outlet requires a 25 foot run of wire from the panel, you need to use this length for any additional dedicated lines even if they are much closer to your AC panel. Make sure the gauge and breaker are rated the same and if possible on the same electrical phase. This will minimize the likelihood of ground-loop related hum.
Best regards,
Grant
Shunyata Research