I downloaded the latest 1.5.1 driver for the Octa-Capture and installed it.  I used it for the first time this morning on some previously recorded material in Sonar.  I was getting some distortion on the singer that seemed to be caused by the bass track, that I never had before.  The Vocal track without the bass track was OK, the bass track by itself is OK, but with the two there was distortion (intermodulation distortion). 
 
I tried several things and then remembered that I had upgraded the Octa-Capture driver.  I uninstalled the 1.5.1 driver, restarted the PC and then installed the 1.5.0 driver.  Loaded the same Sonar project and no more distortion, "intermodulation distortion" or low frequency distortion - what it sounded like. 
 
I haven't tried reloading the 1.5.1 drivers (which I will do at some point) to see if it is the driver. 
 
A heads up, in case someone else runs into the same problem.
 
Jack 
 
***FIXED - A reload of the 1.5.1 driver has fixed the problem..