Happy to try and assist Mark.
I understand your reluctance to do the whole re-install. I have found in the past, sometimes it takes less itme in the long run.
Interesting that good old Audacity works OK. So it must be indeed GB that's giving you the issue.
Lets check this next:
Go the the Finder and launch System Preferences.
Go to the Audio pref pane and look at the Input and Output sections there. Again, I don't have an interface connected, but...
Is your Native Instruments Session audio interface listed and definately highlighted in both the input and output sections/tabs?
Now launch GB and go to your preferences (⌘-, (Command-comma))
Under Audio/MIDI, do you see your Native Instruments Session audio interface listed there under both Audio input and Audio output? This pic below was taken on my MacBook Pro (which I don't actively use for audio recording myself) so that's why I have System Setting listed there.
If you change it, it will probably tell you that GB must change the audio driver, click Yes.
Go across to Advanced. Does your Audio Resolution say "Good"?
Start with that and if sucessful we can try and go higher to say "Better" or "Best", if you want to record at a better audio resolution.
My Console shows this when I launch GB:
com.apple.midiserver: PreSonus FireStudio MIDI Driver - 184.108.40.20675 - x86_64 - Aug 16 2011 05:08:04
It shows the MIDI driver loading (where yours reports un-supported architecture and a 32-bit driver). The previous entries in your log indicate that the 64-bit MIDI is being loaded. (x86_64), but the driver is still 32-bit). I still think that perhaps an updated driver for the Native Instruments needs to be found... Audacity wouldn't care, as it does not support MIDI, but GB does, and would need to load both before being able to record sucessfully.
I'll see what I can find out for you Mark.
If you are using a USB input, make sure you are plugging into one of the USB ports on the back of the iMac, not a keyboard/hub etc.
Check the driver version for your Native Instruments Session I/O. You should be able to check the version number from System Profiler (Applications>Utilities), then click the interface your Session I/O is connected by (USB or Firewire), select your device and see what driver version is listed in the RHS panel's info. I would say it would be 2.4.xx (if your interface isn't that old.)
You might need to un-install all Native Instruments drivers etc first though using the "Mac OS X Uninstaller Utility 1.1.0"
Then install the driver "Guitar Rig Session IO Driver 2.4.23 - Mac"