thyname, not necessarily true. If the biamping results in a different signal level going to the tweeters than to the woofers, then a noticeable difference in sound is not unexpected.

When biamping however, unless you're using an external crossover and have disabled the internal crossover, you should really take care to send the same signal to both sections of the speaker. Otherwise you're negating all the careful research that has gone into making the M80s such accurate performers.