It can be cheaper and a smaller amp. For instance, if you had one or two 8in subs or even a 10in sub that did not require a lot of power (400W max), you could bridge a 200W 2 channel amp to get that. In most cases the 2 channel amp will be cheaper and smaller. Most mono amps put out full power at 1ohm, so a 1500W mono amp might only put out 400W @ 4ohms and a 200W 2 channel amp will put out the same at 4ohms and be 30% cheaper and about half the size. There are only a handfull of mono amps that put out full power at 1ohm, 2ohm and 4ohms. Most put out much less power at the higher ohm loads. I would run a mono amp for anything above 400W but I'm a firm believer in the more power, the better if you adjust the gains correctly.