Being a former amp tech, I used to run  into ground loops frequently from customers. 
 
Having everything on one circuit doesn't make any difference.  Its the "difference in potential of the grounds" that is the problem.  Rack equipment can cause ground loop hums because they are all mounted to the metal rails.  They sell "humfree's" that are nylon isolators to keep the rack ears of the unit causing it isolated from the rails.  Lifting the AC power ground on one piece of equipment sometimes takes care of the problem, but you have to make sure you have a good ground from the other piece(s) or there is a shock potential.   A D.I. box with a ground lift can help in some cases.  In some cases using an Ebtech Hum Eliminator in the signal path can take care of the hum.
 
Finally, if using high impedance signal lines (e.g. guitar cords), cheap cords with poor shielding or using cords with molded ends (not metal like a good Switchcraft plug) can introduce hum.

Do those ferrite deals you clip on to your cables help? You see those built into many wallwarts and other devices with cables.

Those are for "RFI" not ground loop hums.

Another option to consider with this is (if tracking) is a program with noise reduction by noise print capability. If a second or two of "silence" is captured at the ends of each take, it makes for simple destructive edits. There was a thread posted last week mentioning Adobe Audition 3 (there is also a link to the 3.0.1 patch lower in that thread). That has this capability, and can also be added to the Utilities menu for quicker editing if desired.
 
Unfortunately, for performance, this route won't cut it.
 
I took my amp to max power settings for the first time in a long time last weekend and ended up just doing the above (but was just tracking with it). My amp actually recommends not doing this, since it has some excessive signal boosting built in and compounds any noise issues (even minor ones).

Hey guys,
 
I just got my hum eliminator today and plugged it all in, same hum as before... then I hit the lift ground button and heard nothing, beautiful silence!  so thanks to everyone for the help :)
 
And just for posterity it's an Art Zdirect, is a passive box (which is nice, did not need my millionth and one extra cable lol) sucks tho that the first hum eliminator I bought (also Art but over twice as expensive) did not have a ground lift.

hmmm.....

Michelle911
 
And just for posterity it's an Art Zdirect, is a passive box (which is nice, did not need my millionth and one extra cable lol) sucks tho that the first hum eliminator I bought (also Art but over twice as expensive) did not have a ground lift.

Well sheeeyote. I never venture down into this sub forum (for some stupid reason) but for some reason poked my head in today and saw this thread. I too have persistent hum issues AND have been looking for a decent, but cheap, DI/HiZ thingiemadoodle. I was going to try out the MOTU Z-Box but darned if this doesn't look much more suited to my noisy arse setup. Less expensive too. Thanks.
 
http://www.sweetwater.com/store/detail/Zdirect
 
 

Beepster
 
Well sheeeyote. I never venture down into this sub forum (for some stupid reason) but for some reason poked my head in today and saw this thread. I too have persistent hum issues AND have been looking for a decent, but cheap, DI/HiZ thingiemadoodle. I was going to try out the MOTU Z-Box but darned if this doesn't look much more suited to my noisy arse setup. Less expensive too. Thanks.
 
http://www.sweetwater.com/store/detail/Zdirect

Cool, I like Art, I have a 20 year old preamp that's still kickin after some seriously heavy use throughout the years :)

Michelle911
I just got my hum eliminator today and plugged it all in, same hum as before... then I hit the lift ground button and heard nothing, beautiful silence!  so thanks to everyone for the help :)

Glad the issue is resolved.
I figured that would solve it...
I use a Kemper... and when capturing profiles of real amps, there's so much cabling/gear that there's often a ground loop hum/buzz.  Thankfully, the Kemper has ground-lift switches for each input and output...