Tubes are high impedance devices, usually thousands of ohms. Speakers are 2,4,8, ohm etc. The match is made through the output transformer and the ratio is so large that the tubes will not see the passive speaker crossover and try to deliver power even though the driver is not connected when passive bi-amping. This will cause the amp to run hot and be unhappy.
On a McIntosh this 'turns ratio' is 1/4 of other tube amps due to the Unity Coupled output design but still not a great idea.
Ron-C