I do hear noise in my speakers when the amp is on and no music is playing -would this be considered background noise from A/C, or just normal amp noise?
It could be a lot of things, and it's hard to say without more info on your system, and without doing some experiments. My guess, though, is that if what you are hearing is a low level hiss, that is always present at the same level, it is not related to noise on the ac line.
Good power amps are usually pretty quiet, but if you have highly efficient speakers (e.g., 95 db/w/m) that increases the likelihood that the power amp is the source of the noise. If they are inefficient (e.g. 85 db/w/m) the power amp is unlikely to be the source of the noise. Do you still hear the noise with the inputs to the power amp disconnected? Obviously, do not disconnect them while the system is turned on.
Is the noise independent of which source component you have selected, and is it independent of the volume setting on the preamp? Turn off (and preferably disconnect) as much as possible to try to localize it.
Another significant possibility is pickup in interconnect cables, contributed to by inadequate shielding in the cables or by close proximity to power cords or other noise sources.
Also, if any of your components have 2-prong unpolarized power plugs, try reversing the polarity with which they are plugged in.
HTH,
-- Al