I know that the 7x00 series of Outlaw amps are very finicky as far as power coming from the wall goes. I'd say that these amps pretty much require a dedicated circuit with nothing else on it.

Problem for you is to get an amp that isn't as particular about the quality of AC fed to it AND has same or similar power rating can be quite a bit more expensive. I'm assuming that you got a good deal for the used 7500.

I'd suggest using balanced interconnects to go from the new Onkyo prepro to the 7500 once you get the prepro. That should take care of most if not all of the noise being fed into the speakers.

As for the humming of the transformers, if your domicile has noisy power to begin with, even a dedicated circuit won't totally get rid of the hum, even if the amp is behind a power conditioner, so you have to figure out how much hum you can live with.

Either that or get a different amp.