A mono amplifier is a single-channel amplifier, which plays back sound without distinguishing directionality (left versus right, for example). On the other hand, a multichannel amplifier, such as a stereo amplifier, plays sound back through at least two channels (left and right, at minimum). While multichannel amps do produce richer sounds, mono amps can be very useful in certain applications.
Compare price tags. You'll very quickly see that mono amplifiers are usually considerably less expensive than their multichannel counterparts. This is because multichannel amps provide what most people feel is a better, fuller sound quality across the full gamut of frequencies when listened to at moderate levels.
Turn up the bass when you compare the sound that comes out of a mono amp versus a multichannel amp. You'll find that, when bass tones dominate the soundscape, mono amplifiers produce a sound quality that really isn't all that distinguishable from a multichannel amp. This is because the hearing range limitations of the human ear render it incapable of hearing tones in the low bass range in stereo.
Note the impedance levels at which mono versus multichannel amplifiers run. Mono amps always run on 2 ohms, which means they'll be compatible with any speaker system or subwoofer with an impedance that's a multiple of 2 (though extra wiring will be required for 4- and 6-ohm systems). Multichannel amps run anywhere from 2 ohms and up, and thus you'll need to check their compatibility with your other components before buying.
Keep the volume moderate when you're listening closely to compare the distinctive sound qualities that each respective amplifier type produces. The sound refinements audible in stereo are much more difficult to hear when the music is blaring than when it's playing at lower levels. However, no matter what the volume is, you're bound to notice that mono amplifiers have a distinctive, muffled quality to their sound reproduction, whereas multichannel amps have a lighter, richer sound.
Use a multichannel amplifier if you're aiming for surround sound, or if you plan to make heavy use of your equalizer to minimize bass tones when you're playing music in your car. A mono amp is simply not built for surround-sound applications, and the higher quality of stereo or multichannel sound is even more evident when treble levels rise on the equalizer.
Choose a mono amplifier if you're going to be pumping a lot of bass and you have a relatively simple set of speakers. This will save you some money, since there won't be as dramatic a fall-off in sound quality when bass tones dominate the sound.
