The more equipment you add the more potential for noise. I hear even small commercial studios that have an unacceptable amount of noise. I did a lot of research on noise a few years back. The BIG studios often have a grounding spike driven 20 feet into the ground. Of course that's too expensive for most home studio people.

So the next alternative is to hook everything up on the same electrical circuit (or better yet the same OUTLET). Before you try this you need to add up the maximum wattage that you will be running at once so that you don't overload the circuit and trip the breaker or start a fire.

Note that a piece of gear that is TURNED OFF can be adding noise to your system. I have a keyboard module that only causes hum in my system when I turn up it's volume on my keyboard submixer (and this hum can be duplicated when the module is turned off).

Really it's a process of elimination. Your problem may be as simple as ONE or TWO pieces of gear that are causing the noise. Do a process of elimination and unplug gear one at a time; and sometimes you have to unplug 1/4 inch cable and midi cables too). Isopatches can help but often don't. Don't use ground lifters because if you were to unplug it's 1/4 inch and midi cables from your other GROUNDED gear you could potentially get shocked.

Sometimes it has nothing to do with your gear. I had a situation with lighting. try different lights maybe. Florecents are a no no. I was told of another situation where the entire studio was went through to find out is was his computer monitors. Some older (non- flatpanel) monitors will cause noise. Also tv's / security screens are known to cause noise. Hope this helps.