The only variable in my troubleshooting mix was the Amp Head. All my existing configuration setup with same room in my house, 3 different amps, outlets, 3 cables, 3 basses, etc. didn't have this hiss issue. I could be wrong of course but I isolated it to a one-off squirrely amp.
If so, then a replacement head that exhibits the same noises would indicate that it's not the amp, but something in your house wiring, or some part of your entire signal chain. That is still much more likely than a "one-off" head problem. Mesa has strict quality control on every head they build and send out.