Finally got latency down to the point where I can record.
 
So I added a clean lead guitar track ... so far, so good.
 
But when I add effects (using either Pevey Revalver or some amp sim bundled in with Sonar Producer) the other channels become distorted.
 
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% and still 25% of RAM to spare).
 
So what would cause effects on channel 8 distort channels 1-7?

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.

Are all the channels feeding into a buss that is overloading? Your description doesn't say if you are observing clipping on the meters or hearing it on playback.

Which interface are you using and with which driver?

Amp sims use a lot of CPU. You mentioned getting the latency down to where you can record...it may be that an amp sim tips things over the edge. If you increase the latency and the distortion goes away, you'll know the cause.
 
Check out Friday's Tip of the Week for Week 35 for a potential way to improve latency performance.

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.)

So many good questions and insights. Let me try to address them in one pass:
 
CPU: The CPU load is low, less than 25%.
 
BYPASS: When I bypass the FX bin for just the lead guitar, the other instruments regain their clean sound.
 
CHANNEL OVERLOAD: I checked and I was over driving the mains a little, but when I backed that down the other instruments sounded as if they were being fed through a distortion pedal. 
 
DRIVERS: Using ASIO and I did up the latency to see if this helped, but it didn't.
 
What makes this so odd to me is that before I worked to get latency down, I would use the same FX tools and never had this problem.

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.

Are you using the supplied drivers or a generic ASIO4ALL one? Are they updated? Which interface are you using?

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?