Passive Bi-amping
Passive bi-amping utilizes the passive crossovers built into the speakers, with each amplifier channel reproducing a full range signal to drive separate high and low frequency networks. Relative to active bi-amping, the benefits of passive bi-amping are much less pronounced (you might hear the derogatory term of “fool’s bi-amping” applied). Nonetheless, splitting the high and low frequency networks does have some effect, as the amplifiers each now see a different load than one would under normal circumstances when individually driving the entire speaker. Generally speaking, the individual networks are designed such that “out of band” frequencies will have a very high impedance relative to the expected pass band of the driver(s), and consequently will demand significantly less actual power from the amplifier at those out of band frequencies.
Because of this split, there are a couple practical advantages. Potential output ability is increased as there is little power wasted reproducing those aforementioned “out of band” frequencies. As with active bi-amping, the possibility of tweeter burnout due to amplifier overload is also reduced. However, in terms of basic audible differences, there’s not much to write home about, and what improvements do exist could typically be gained by simply utilizing a single more powerful amplifier with similar or better performance metrics than the two smaller amplifiers.