Ohms

The reason people think they need to do run amps at the lowest possible load is that a lower impedence (ohm) load will place more current demands on the amplifier.  More current = more watts (power.)  So by running an amp at its lowest safe ohm load, it will operate closer to its peak output capability.  Translation: you can squeeze a bit more SPL out of the equipment.  Running amps at or near their lowest ohm load is an SPL trick.

The down sides, and there are many, include the following:  NOISE, HEAT, SHORTENED LIFETIME among others.  An amplifier operating near it's maximum capability is much noisier, produces much more heat and will not last as long as one operating in the middle of its band.  So, for SQ setups, I design the system so that the amplifier will never be required to oeprate at more than 3/4 of it's RMS capability for the cleanest output.  Also, it is good to design the system with speakers rated at 2X to 5X the RMS power of the amp so they will not drive the amp into clipping, or be damaged by the power input.