Volume level for slightly overpowered amp


I have bought an amp that outputs 125 watts per channel, connected to speakers that are rated for handling 100 watts per channel and recommend amplifiers of 40 W to 100 W.

I understand that this is no problem as long as I don't crank up the amp's volume too much. But my question is, how much?

If I understand the amp's volume setting to be linear with the number of watts, then I would be able to crank it up to 100/125, i.e., to 80%.

But I have also read that perceived volume is not linear with watts (large differences in watts create relatively small differences in perceived volume). Then if the amp's volume setting reflects perceived volume, it wouldn't be linear with respect to watts either, so I probably would be able to set it to a bit more than 80%.

My question is, which of the two assumptions is true? How far do you think I can move the volume setting while being safe not to damage the speakers?

Note that I'm not crazy for a bit of volume. I mean, if the answer is "80%" I'll probably not go over 75%, if the answer is "90%" I'll probably not go beyond 80-85%, just to be on the safe side. But I would know what the safe side is a bit better if I knew where I stand, anyway.

It's not power in watts that kill speakers it's distortion...I have fed tiny 3.5" Energy speakers with 150watts & the sound those little suckers made was amazing...I imagine your ears will give in before you hurt the speakers....Party on!!!
You must also consider the source input voltage. The volume setting that drives the amplifier stage in an integrated amplifier to full rated output will depend on this. With digital sources this often occurs prior to the volume control being 80% open.

Your speakers should 'tell' you when they are being overdriven. 

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Power in watts vs volume knob position is not linear - it is exponential (since human hearing is logarithmic).  When you set volume pot at 50% of the peak volume position amp will produce 10% of max power (1/2 of volume = 1/10 of power).  Assuming 100/125 ratio of power is equivalent to about 94% of the volume* it will be very close to max power position, but we usually don't know what it is.  Probably most of amps have max volume between 1 and 3 o'clock position of the volume knob to provide additional gain for softer than nominal sources/recordings.  I agree with Elizabeth - 600W would possibly be a problem, but you would hear huge distortion before burning down the speakers.  Slightly larger amp's power rating is even healthy since most of the speaker damage is done by underpowered amps that clip when overdriven.  Clipped signal has a lot of harmonics and tends to damage the tweeters, that are not designed to handle it.

* Perceived loudness = k^(1/3.5)  where k is the ratio of power.
Listen. Pay attention to the start of distortion. That's when either the amp or speaker or both are being stressed.