B&W CM1 bi-amp impedance

See Chart 5 at this link:

http://www.soundstagenetwork.com/measurements/speakers/bw_cm1/

As you can see the impedance varies all over the place, but 8 ohms certainly looks best for the tweeter. A lot of the woofer range looks like it's about 5 or 6 ohms, rising higher at some frequencies. Not sure why your woofer amp would sound distorted when connected via the 8 ohm tap.

Keep in mind, also, assuming your amps are identical models, that if you connect the woofers to the 4 ohm taps of one amp and the tweeters to the 8 ohm taps of the other amp you'll probably be introducing a small volume imbalance between woofer and tweeter (in the direction of lowering the woofer level relative to the tweeter level).

Regards,
-- Al

Robrob, I think you answered your own question.

"In McIntosh's owners manual it says it will cause distortion if you do not choose the right impedance.Tweeter amp sounds good at 8 ohms, but woofer amp has distortion."

Try both and see what sounds best.