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1) one four channel amp
Cheap, effective.

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2) two two channel amps
Added expense for little to no improvement in performance.

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four one channel amps
Even pricier, takes up more space, requires more wire/wiring hardware, difficult to find a mono amp that is not class-D.

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two four channel amps bridged
No advantage over a single four channel amp.

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5) four two channel amps bridged
too much money, no advantage.