Both the speaker amp and the headphones are connected to the comp by standard 3.5mm aux jacks into the computer motherboard.The MOBO is a GIGABYTE GA-P55A-UD3 LGA
On the front, there is a bank of 6 different colored ports to plug the jack in all right next to each other.
The headphones and the amp are each connected to one of the jacks. I am going to turn the headphones up and see if the noise is there, just not as noticeable. If it is, then it's not likely to be the A/C power that's dirty, but instead some interference making its way into the mobo from the powerline. Btw, my amp is something like "Pyle Home PCA4 Mini 2x120 Watt Stereo Power Amplifier" (i'm not at home so I don't know the exact power rating, but it's that base model).
Nothing fancy, and I've never had any issues with it until now. The noise is present whether or not any music/sound is being pushed through the speakers and is independent of the volume setting on the amp.
**Apologies for the large images. I tried to link to the product pages, but got a notification saying I can't post URLs.