Amplifiers

Power amplifiers are classified according to their design. In audio the classes A, B, AB, C and D are used.

The subject is very well covered here.

Class D is very efficient, more then 90% of the power is used for the music signal. In case of Class A 50% of the power is dissipated as heat..

A modern Class D amplifier can deliver 320 Watt at 4 ohm in an almost Mac Mini size package. If global warming (or a low electricity bill) are your concern, these are the ones to go for.

Source: Nutshell High Fidelity

Bi-amping

A lot of speakers have two pairs of binding posts, one for the woofer and one for the mid/high.
This allows you to drive the woofer and tweeter with separate amplifiers.

A much better solution is to drive each speaker with its own matched amplifier and do the crossover on the signal before it enters the power amps.
This allows for better filters than possible with passive components in the power part.
A lot of studio monitors are build this way.
This is called an active crossover.