Izotope RX4.  A little expensive, but works great.  And can be used in all sorts of situations, not just harmonic hum.

Beepster
I'm currently tracking but there are quite a few posts I'd like to respond to and I've done some research so have some thoughts for this thread and Craig's mastering thread but I just wanted to quickly address your post, rcees.
 
First of... hiya. :-) Using R-Mix on the bus would, in theory, make sense but what I'm actually trying to do is yank out the hum before it hits the sims on the tracks (the sims are in the FX bins of each track). It's weird because the dry guitar signal actually has no apparent noise on them but once they hit the sims they do. Putting R-Mix (and/or Sonitus set to the 60 hum removal preset) after the sim on the track though is not as effective as having it before the sim. I'm basically stopping whatever is triggering the hum within the sim BEFORE it hits the sim. Kind of like I'm grounding my guitar before going in to an amp/stompbox if this were a hardware setup. Putting it on the bus would obviously not work for this and I like to have the sims on the individual tracks so I can set them up different to be complimentary to each other (two inputs with different tones so different effects custom tweaked).
 
Also the bus is actually being fed by three sets of two tracks (there are harmonies and doubles so 2x3 = 6 tracks). Getting rid of the noise on the tracks seems like the best course of action.
 
Just wanted to explain what was going on. However I have gone for a different approach where I am using a blend of the two channels on the amp but I also realized this morning that even when I use the channel blending option that that signal blends into the first of the two dual outputs BUT the second channel still gets output through the second output (without the first channel's signal). This means I have a less powerful output on that channel but it is still something extra I may be able to use to beef things up or pan or do mid side or whatever.
 
I could actually take all this even further and split the guit signal using my TU-2 pedal and send into my other amp as well to create three signals but I don't think that is necessary and splitting in this manner unfortunately seems to introduce a bit of extra noise from the pedal (or bad cables or interference or whatever).
 
Anyway... thanks all for contributing. As always it has gotten me thinking about some important stuff. Today is all about tracking though so my apologies if I don't return right away.
 
Cheers.

I tend to clean any problems in an audio track then render it out (bounce) so that I'm working with clean audio files to begin with. It makes any following fx much easier to set up.

Beep,
 
In an effort to get more out of R-Mix > 
Have you considered creating a phase flipped clone of the track and then using R-Mix to get only what you DO NOT want on the clone then use the volume automation to bring it up against the original cancelling what you don't like?
 
I found that this approach really helped me leverage noise cancellation with the Melda Tools, so maybe it would help using R-Mix.
 
You could also consider using a side-chained Gate or compressor. Send the original into the side chain on the cloned and phase phlipped track and suppress the noise cancellation when you have significant volume on the original.
 
Just thoughts. Seems to me like I could do a lot of cool stuff with R-Mix. I just played with it in SPlat and it worked ok for me.