Running at too low an impedance will be a problem for the power supply. It will certainly TRY to run with more power, but what WILL happen is that the SMPS in the amplifier will top out. The amp will try to demand more power, but what will happen instead is the power supply voltage will droop. Generally the fuses will go before the amp does, if the fuses are well matched to the power supplies actual capability, though now a lot of people have clued in and check the fuse ratings on their amps for a real idea of how much power the amp will take, so many manufacturers are putting in overspec fuses, with the result that the switching transistors in the amp power supply will be in serious danger.
If you can keep the amp cool enough, the transistor can handle more power, transistors are rated for an amount of power at a certain temperature, and are derated as they get hotter.
The problem is that most newer amps aren't using regular transistors in the PSUs anymore. They're using MOSFETs, which derate in a completely differnet manner. Unlike transistors, MOSFETs have to get very hot before they start to derate signifigantly, so the manufacturers don't overspec the devices nearly as much. The devices are desirable because it is much harder to get thermal runaway with them (Where they get hot because they're hot.) Some say it's "impossible" though it isn't -- it's just far more difficult.
The problem though is that with these devices that aren't as overspec, if yoiu try to draw too much power through it, cool or not, it'll just fail. Then you've got no power supply, you've got no amplifier.
