I have 50 watts out put on my home Hi Fi and run at around 5 or 6 if you think in terms of 10 being max . I can't visualise a situation wher you would WANT a home system to be so loud that a Forty watt speaker would be harmed , unless you were going to have a teenage party and really wanted to annoy the neighbours . The Impedance is more likely to cause a problem with the final stage of your amplifier , but it seems , at least over this side of the pond , that most home system speakers are 8 ohm .