For hum related issues, yeah, HumX is a good thing. So is a good power conditioner. For hiss related issues, as others have said, optimizing your gain staging is best and a noise gate is easiest. For higher gain sounds, I usually use both (making sure all my levels are as high as they can be without clipping at every junction in the audio chain).
Since pickups are magnetic in nature, anything with an electro-magnetic field can (and will) cause hum (like computer power supplies, the transformers in guitar amps, fluorescent and neon lights, etc.). Motors on the same circuit (like washers, dryers, HVAC, dishwashers, etc.) don't even have to be close as they can cause ripple current in the AC line itself. This is where products like HumX and good power conditioners can make all the difference in the world.
In my case, the biggest problem with noise that I used to have was just having the computer fans kick on at the wrong times. My solution was to build an isolation cabinet for the computer (I put it in a rarely-used bathroom and covered the door with an old blanket, after making sure that the air conditioning vent was open so it wouldn't overheat). I had to buy longer cables for my monitors, mouse, keyboard, etc., but it eliminated that problem.
Just as an aside, I used to play this one bar that was up stairs from a furniture store on one of the main drags through town. Every single freakin' time a cop drove by, his radio would start coming through my 412 cab. Tried everything I could to stop it, but I was always completely unsuccessful. LOL! It ended up just being one of those oh-so-true Spinal Tap moments. After we played there a few times, even the patrons would laugh about it. At least they would've had advanced warning if there was an impending raid on the place!!
Had some great times there.
