100% it's your electricity. What you're hearing is the hum of your hard drives on your computer and just the general planet earth ground hum at 60hz.

I had the same problem with my Virus A. What you have to do, is find a different circuit to plug your virus into (one that does not have any computers, TV's, microwaves, or LCD monitors), or, get the 3-prong to 2-prong power cable adapters and put them on everything in your studio that has a 3-prong plug.

This hiss and hum, is a ground loop. I spent months trying to figure it out and came up with two solutions. Spend $200 on a power conditioner to plug my computers, screens, and speakers into, or buy a dozen of these power plug adapters for about $20. I started with the cheap solution, and when I could afford it, I bought a Furman power conditioner for the studio.

The only downside to the cheap way, is that you're removing the ground loop on the instrument, which in reality could make the chassis of the synth electrically conductive, which could cause it to get damaged if there was ever a power spike in your circuit or it could just shock you for the hell of it, especially if the paint is chipped and bare metal is showing.

also, check your preamps to see if they have Hi-Z instrument input mode versus Mic/Line. This could help as it puts a db pad helping to remove the noise floor.