mettelus
Start with "All plugged into various power strips on three different outlets" using a minimalist approach and go from there. You want to build up knowing you are on a common ground, then analyze environment.

Power with a single outlet where you can to minimize the potential for ground loop hum.  
 
All gear should go to a single outlet if possible.
 
 
And plugins like izotope are way better than trying to carve out said frequencies at the source.  It is able to attack very specific things unlike the EQ on your amp, or some piece of junk noise gate pedal.  (if you can't fix the hum electrically)

Also, just in case one of those outlets is on a circuit with other equipment, it would be worthwhile to check all three outlets (one power strip, minimal gear). Pulling a ground hum from multiple outlets is highly likely, but you may also be plugging into the refrigerator circuit by accident only trying one.

Everything plugged into a "single outlet" does not guarantee no ground loop hum problems.

fireberd
Everything plugged into a "single outlet" does not guarantee no ground loop hum problems.

If that single outlet is properly grounded, and you don't have other power supplies along the way, it should - assuming each device plugged in has proper grounding in the electronics.
 
If all those criteria are met and it still is humming, it isn't a ground loop hum.  Not all hum is ground loop related.

Some hum and "blow" from an amp is pretty much unavoidable. The trick is to try and keep,it to manageable levels.

As well as everyting else suggested, a big part of controlling noise is the amount of gain added to the signal and where that gain is added. An inherently noisy fuzz, for example, is going to produce far more noise at the speaker if the amp is set to high gain than if the amp is clean or almost clean. So the first thing to address is how much gain is necessary at the amp in the first place.

If you're not using any pedals after that it comes down to the guitar. Sometimes just turning so you face a different way can remove an awful lot of EMI/RFI from single coil pickups. Even vintage-style Tele pickups are pretty high output for single coils, which makes them noiser that e.g. a vintage-style Strat pickup. P90s are even worse. Teles are my favourite guitars, but for high gain stuff I generally use an SG.

I prefer not to gate noise out if at all possible, because it always costs sustain and often the noise floor increases to unacceptable levels while the note is still sustaining and you want to keep that sustain. Having said that, BozDigtal's Gatey Watey is a gate that only gates out particular frequencies so if the problem is mains hum and its first harmonic (50/100Hz in the UK, 60/120Hz in the US) it can often reduce those frequencies without affecting the rest of the signal too much. It might be worth a try. Another approach is to use an eq with very tight Q settings to place narrow cuts where the hum spikes appear.

White noise is harder to deal with, one solution that might help is to replace the 1st pre-amp valve on the amp(s) with ones that make less noise, that being the valve that gets amplified most by the rest of the circuit. The difference in noise production between even supposedly identical valves from the same manufacturer can be surprisingly high. Even vintage NOS valves from manufacturers such as Mullard, Phillips or Sylvania can be noisy and they do vary in the noise they produce. In current production, JJ and Tesla are pretty reliable for not making too much noise, and the cheap Chinese valves favoured by some amp manufacturers seem the worst culprits. A valve meeting the 7025 specification is likely to be less noisy than "ordinary" 12AX7s.

Or drop the amp's preamp gain a bit until things quieten down.

Where pedals are involved the only answer is to select them carefully, based on noise level as well as what they do, then run the pedals and amp so the pedal's don't raise the noise floor too much. For example, running a particular cranked germanium fuzz face into my Tiny Terror with the amp preamp gain at noon results in a 15dB increase in noise floor. Turn the amp gain to 1'O'Clock and the noise floor is raised by over 30dB. Backing off the fuzz's "fuzz" control a bit pushes the noise floor down a lot without making any noticable amount of difference to how the pedal sounds or what it does. Higher cost doesn't always equate to less noise, two of the least noisy overdrives I have are a Mooer Rumble Drive and Hustle Drive. The Hustle is less noisy than my OCD.....

Well,  being a simple guy...  I think we have to find it before we can reduce it.  I have many amps, many effects, yards and yards of cable,  many guitar types,  many other system components such as pre-amps, phantom power suppliers and the like. Sometimes you have to start at the beginning - and this is an over simplified process.  You may have to dig deeper into a step.   
 
Step one.  Unplug everything in the room.  This will be a isolation starting point and help to protect your gear for step two.
 
Step two.  Go to the circuit breaker and find the breaker for the room.  Turn it off.   Check to see it affects only that room.  If things go off in other rooms, unplug the stuff in those rooms.  Turn it back on.  We need to find out if that circuit has other items on it.  While this doesn't isolate us completely, its a starting point. 
 
Step three.  Beg, borrow or buy a inexpensive circuit tester - one that has the ability to check for open ground, open neutral, open hot, hot ground reversed, hot neutral reversed.   Test the circuit.  If it fails,  then have it repaired by a qualified, trained and insured professional. I do it myself - but I'm a nut case. 
 
Step four.  Plug in one amp.  Set the gain to your liking.  Find a known working guitar cable, plug the guitar in.  Note the noise.   Note your seating position and use this position for all other tests. EMI\RFI patterns can build up all over the room depending on the density and type of the electronic 'toys'.   If you fail with the amp alone and a good working grounded circuit, it could be the amp or the entire energy source.  I've had both happen, more than a few  times.  If its the energy source, it could happen intermittently or time of day.  I hate when that happens - somethings putting noise on the line ahead of your house and that tends to be the hardest to fix because its out of your control - call the power company.  Guys who perform for a living know all too well the variability of power quality - they try to afford power conditioners and voltage regulators for a reason. 
 
Step five - plug in your guitars one at a time and listen so that you have a reference for each type and quality of pickup you have.  This noise will be enhanced greatly by the pedal board step below.  Try plugging in each instrument cable you use.  Listen closely - these little hoober doobers are great EMI\RFI antennas.
 
Step six.  Plug in your PC, monitor, 'sound card', playback amp, speaker and microphone with good known working cable.  Turn them on.  Sit in the 'test' position with your guitar and amp running - you will use this position for the entire process - by staying in one position, you are testing for the combination of EMI\RFI buildup. Note - PC's and some monitors emit a healthy dose of noise.  One of the posts points that out - you can be sitting three feet away with pickups pointed towards the PC - monitor.  Turn away by 180 degrees and the noise drops noticeably.   Record.  Everything OK still?
 
Step seven. Testing pedal board components - power supply, cables, wall warts, effects units.  A healthy dose of both EMI\RFI injection (adding noise) and reception happens in this area.  In my personal experience, a large portion of 'NOISE' or buzz comes in here and it can take hours to shake out.  
 
As a point of reference, I run a Voodoo system - everything is racked, and my cable runs are inches, my cable is the best quality I can afford, I have each cable made by a really good sound company out where I live - they test for capacitance and noise rejection (anal, I know),  I listen to each effect as I am adding new ones to 'know' the buildup of noise they add, I use a power conditioner\ voltage regulator that the entire system is plugged into, I also use a quality power supply for the pedal effects.  I am noting this to show you can go to expensive and great lengths in the chase for eliminating noise.  By the way, wall wart devices that are digital tend to have a propensity to add noise - and a cheap power supply can transmit noise between devices. 
 
Plug in one effect - cable it up, plug it in to its power supply and listen.  You are now adding both noise injection and reception - you will get a buildup, and note it. add the next effect and so on, stopping to note the noise.  If this step passes - SUPER!!  Most of my failures come here or the next step.
 
Step eight - add another amp(s) one at a time.  Don't cable it in, just turn it on.  Sometimes amps have issues where just running a second one on the same power leg can cause hums and buzzes, even when they are not connected together via the effects pedal.  OK?   Then cable it into the pedals.  Note - you may be running effects in the effects loops of each amp.  Those effects could be plugged into the same power supply and a noise from one amp is injected into the other amp just from power supply association - happened to me with a set of eventides, and those are designed and built by a QUALITY company. They didn't add the noise, the one amp interacted with another amp through the devices, via the power source.  An easy way to get a bit of noise is to connect both amps effects loops into the same effects device (stereo).  
 
Step nine - my guess by now you've found that you have a sum of things causing the buzz or noise.  To finish it off, start turning on the 'other' things on one at a time, in that room and any room associated to the same circuit...
 
You should have been able to isolate the exact point, or become familiar with the summing of noise that you are hearing through the entire chain.  Theres a lot more you can do - it usually just comes down to a simple test of isolation, then adding, then listening to the sum of the components at each point..
 
Take care..

Brian Walton

fireberd
Everything plugged into a "single outlet" does not guarantee no ground loop hum problems.

If that single outlet is properly grounded, and you don't have other power supplies along the way, it should - assuming each device plugged in has proper grounding in the electronics.
 
If all those criteria are met and it still is humming, it isn't a ground loop hum.  Not all hum is ground loop related.

I use the same approach as Brian. Run from the same outlet when ever possible. When you are using equipment that will be plugged in to different outlets (and that equipment will be connected together) using alternating current, you run the risk of a ground loop.
When you power connected equipment from the same AC socket it eliminates most ground loop problems.
 
This is a NO

 
This is proper

chuckebaby

Brian Walton

fireberd
Everything plugged into a "single outlet" does not guarantee no ground loop hum problems.

If that single outlet is properly grounded, and you don't have other power supplies along the way, it should - assuming each device plugged in has proper grounding in the electronics.
 
If all those criteria are met and it still is humming, it isn't a ground loop hum.  Not all hum is ground loop related.

I use the same approach as Brian. Run from the same outlet when ever possible. When you are using equipment that will be plugged in to different outlets (and that equipment will be connected together) using alternating current, you run the risk of a ground loop.
When you power connected equipment from the same AC socket it eliminates most ground loop problems.
 
This is a NO

 
This is proper

So would daisy chaining multiple power strips but a no no even if they were coming from the same outlet?

I've seen problems where two amps are plugged into the same AC power strip (same outlet) and there is a ground loop hum problem.  I've seen an effects processor, plugged into the same AC power as an amp cause ground loop hum.  
I've seen several pieces of equipment in a rack, plugged into same outlet, have a ground loop problem caused by one of the devices (many times isolating from the rack rails will fix this).
 

chuckebaby
This is a NO

I've a setup similar to that using two wall sockets off the same room ring and a bunch of extension strips which produces no measurable noise at all.....

I am however careful about what gets plugged in where and if it creates noise I shift it round sockets until I find one where it doesn't create noise - or even decide not to use whatever it is. There's no switched-mode light dimmers on the circuit either, which helps a lot.

On the subject of switched mode wall warts, some guitar pedals really hate the things, even modern digital pedals. And sometimes psu noise gets transmitted to other pedals that aren't even getting power from the offending wall wart. Then there are the pedals that are noisy unless placed after a buffer - including the many analogue modulation effects that leak clock and click away even when "off" unless getting a low impedance signal. And there's the effects that hate being after a buffer....