Interesting to note that, even if you have less impedance, say you connect 4 ohms in an 8ohm exit, you will never get rated power.
Imagine the equivalent(simplification) of your circuit
and your speaker being the load (forget the values and that they're only resisitive).
You'll get max power transfer when Rload=Rthevenin, else you'll always get less power.
If you mismatch your impedances, probably nothing happens, unless you start to crank your volume up.
If the equipment is new, it probably has the necessary protection, but I would avoid it anyways. Doing it would probably shorten its lifetime, and increase the heat.
So, although it may seem odd, if you put higher(or lower) impedance, your power consumption will be higher for a given volume.