If we examine, two and three way passive loudspeakers, it is apparent that some manufacturers facilitate for the connection of more than one amplifier. This multi connection concept is known as bi-amping or tri-amping.
A loudspeaker is able to be bi-amped when you have independent access to the tweeter and bass mid drivers. This is typically made possible by removing a binding post link between tweeters and mid/bass drivers. The same applies to a tri-amped situation where you can physically separate the tweeters from the mids and the bass drivers.
Once you have ascertained this aspect, you are free to experiment with bi- and tri-amping your audio system.
System relativity is an aspect worth your serious consideration. Say you have decided to spend $20.000-00 on a pair of high quality loudspeakers. Surely you will want to ensure that they are amplified in the optimal manner. Bi-ampling and tri-amping solutions are therefore a logical step to explore.
The tweeter typically uses only a small amount of the total energy requirements of a loudspeaker. As such it consumes only a fraction of the total output power coming from your amplifier. And yet, whilst its requirements may be insignificant, it performs a very delicate and important role in your system. Why then risk neglecting this aspect and why not supply it with its own amplifier, freed from the constraints of having to supply the comparatively massive amounts of energy to drive loudspeaker woofers.
Assuming you have independent access to your tweeters, mids and woofers, bi and tri-amping topologies will improve almost any audio system on account of dramatically reduced levels of intermodulation distortion and the reduction of distortion caused by the bass driver passive crossover network.
Horizontal ….. Utilise independent amps to drive the woofers, mids and tweeters.
Vertical …… Takes a different approach. In vertical, one chanel of one amp will drive the bass drivers of one speaker and the other channel will drive the mid/tweeter cluster.
The horizontal approach, which facilitates for different sized amps being assigned the task of driving different sized loads. Big amps used to drive woofers and smaller amps for mids and tweeters.