In addition to the experiments that Ralph (Atmasphere) suggested, if readily practicable it may be worthwhile to try running the amp with its top cover removed, which would lower its internal temperatures.
Regards,
-- Al
You would suggest I get 8ohm speakers?If you are happy with the sound of the speakers during the first hour, and if the conclusion is reached that either the speakers or the amplifier has to be changed, it would probably make the most sense to either change the amplifier, or to purchase a separate power amplifier and drive it from the M6i's pre-outs.
Regards,
-- Al