When should I use a mono amplifier instead of a multi channel amplifier?

Because mono amps tend to be Class D amplifiers, they are a good choice for powering subwoofers — Class D amplifiers have a high power-to-heat ratio and excellent efficiency, which are exactly what you want when dealing with power-hungry low frequency signals.

Most mono amplifiers are designed to run at 2 ohms; some are even 1-ohm stable. Multi channel amplifiers, on the other hand, are typically designed to work with a 4-ohm load. This is an important difference when using your amp to power multiple subwoofers, because you won't be able to bridge your multi channel, 4-ohm stable amp to power multiple subs that present less than a 4-ohm load. Instead, use a mono amplifier to power a 2-ohm load — two 4-ohm subwoofers, or 2, 2-ohm dual voice coil subwoofers, for example. You'll be able to push your subwoofers with the mono amp's maximum power, without running at a dangerous impedance.