Hi all!
After some headache I discovered that Channel Tools introduced distortion, as if there were some bad quantization.
I am NOT a professional.
It was a choir, I made the recording in a Church, with a stereo pair of Mics at 24/88.2 (88.2 kHz, because it goes to a CD, so I thought it would be better at quantization time)  but I accidentally swapped Left and Right channels (did I tell you I'm not a professional? ;-)).
Now in the mixing phase I dropped a "Channel Tools" in my chain to swap channels again, but I here a bad distortion on the sopranos' forte. Not so evident at the first auditions, but it is there.
And It goes away bypassing Channel Tools...  May be that it doesn't support 24/88.2 ?
Now I used Process | Apply Effects | Gain...  to definitively swap L/R channels (and I promise I'll never ever do such a mistake).
Anyone had similar problems? 
I can't trust Sonar's audio chain anymore if I can't find a reason for that...
I can assure that the original recording sounds good.
Mics: Neumann U87A pair, ORTF
Pre: Mackie VLZ3
A/D interf.: Echo AudioFire 2
Recorder: Sonar X2 Producer.
 
Thank you, and sorry for my poor english.
Francesco

My initial guess is that the levels feeding the Channel Tools (CT) are too hot. Try reducing the "Gain", as that is pre-FX. Can't remember if CT has input level adjustments (probably does), but if so, you can also lower the levels there. CT adds no distortion as long as there is no clipping and you do not have any of the delays enabled. Enabling the delays may add audible noises depending on the material and delay settings.
 
If reducing the levels doesn't solve your problem, make sure there are no other plugins or Pro Channel settings that might be clipping.
 
Regards,
Dan