Other channels distort after adding guitar effects

guyshomenet
At first I thought it might be I was over stressing the PC, but both CPU and memory are well below thresholds (CPU ranging 10-25% ...

I haven't done the setup you are trying (with the amp simulations), but the CPU usage seems really high.  Do you have a bunch of other tracks with lots of effects?  You may have to freeze some of those tracks.

Which interface are you using and with which driver?

Craig, in your Week 35 tip you have a red box around the Realtek driver (which is shown as disabled) but also the AMD HD audio device, which is NOT disabled in your screenshot.
 
If I look in the same place on my DAW, I also have a Realtek (which I now have disabled, same as yours), and similarly a "Intel(R) Display-Audio".
Are these also supposed to be disabled? (I ask, because your equivalent driver is NOT shown disabled.)

guyshomenet
CPU: The CPU load is low, less than 25%.

That isn't necessarily low.  If you have a quad core machine or maybe a dual core with hyperthrreading, that may mean that one of the cores is maxing out.

Again, many thanks to all. I may have this narrowed down a bit more.
 
CPU: Actually, my CPU was higher than I thought. Using Windows Task Manager, it was more into the 50-60% range.
 
MULTI-CPU: One of the tips I read for reducing latency was to disable the Multiprocessing Engine (which is counterintuitive). I reenabled it, though doing so had only a slightly positive effect.
 
ASIO: Using the ASIO driver that Lexington provides for the Omega interface. I have not tried ASIO4ALL (any thoughts about its viability over Lexignton's).
 
AMP: While experimenting, I clicked the power switch icon on just the amp in the TH2 Producer stack and all the other tracks cleaned-up 90%. Adding a distortion pedal did not make things worse. I get similar results when bypassing the amp in the Pevey Revalver stack. So it appears the amp modeling is the primary suspect, though it is unclear to why. The CPU load doesn't change much with them on or off. Are they inducing some degree of latency that causes the equivalent of clipping on the other channels? Is exporting audio (preliminary mix down) a good test for this?

guyshomenet
ASIO: Using the ASIO driver that Lexington provides for the Omega interface.

You do mean Lexicon (version 2.7) not Lexington, right? Lexington does not provide any drivers.

Yes, sorry ... Lexicon (brain fart on my part). ASIO drive 2.7.0.

Additional diagnostics:
 
I cut to MP3 exports, one with the amp simulator and one with the distortion pedal (see notes above). The same degree of cross-channel distortion was present in the MP3 files.
 
Perhaps there is something I don't understand about Sonar, but I would think that rendering the MP3 files would not have real-time, latency dependent issues. Since the induced cross channel distortion occurs in real-time (output over the Lexicon) and in the MP3 file, I think something else is afoot.

You don't describe the type of distortion, but I wonder whether this is connected with your OS's Power Options. I find that Sonar glitches very easily if any degree of Processor Power Management is engaged. Go to Control Panel - Power Options and ensure that you are using a High Performance option. If you want to go under the hood look for Minimum Processor State and make sure it's set to 100%.

I'm not convinced that Sonar is not implicated (at least in my case) although to be fair I rarely run Sonar without audio. Other software (albeit less demanding) does not seem to create the situation. That said, it is no big deal. I have Power Options pinned to my start menu, so it is a matter of seconds to switch to flat-out if Sonar (is involved in) glitches. The glitching immediately stops - I don't even have to stop the transport. I made the suggestion to OP largely because it is so easy to try, and if it solves OP's problem that would be great.

ASIO: Using the ASIO driver that Lexington provides for the Omega interface.
 
This. Sorry, but there has never been anything good said about these drivers. 
A new interface with trusted drivers is you best bet. 

Two thoughts:
 
Rendering may be happening in real time or not depending upon how you have your settings. Also, I think some plug-ins only support real-time processing.
 
What happens if you mute the track with the intended distortion during playback? The mute does not prevent the CPU-load from the processing of the signal. It only prevents the processed signal from getting through to the final mix. If the unintended distortion does not go away, that would suggest it is something to do with CPU contention or overload of some kind. If it does go away, that would suggest some kind of intermodulation effect, which would make me look very carefully for unintended signal routing between tracks and buses.

^^ To which I would add also try the MME drivers, which is your basic lowest common denominator audio drivers. When troubleshooting always try the simple stuff first.

Balls.

What's your processor and amount of memory?
Have you followed the Friday's tip of the week that related to removing extraneous drivers that degrade performance?
 
Guitar amp sim distortion traditionally draws a lot of CPU. At this point I'm beginning to think either your computer is underpowered, or something is compromising its power.