Irv, thank you for offering the words of reconciliation to Grant.
03-16-11: Irvrobinson
For a good voltage-source amp with 100w into 8ohms, if the speaker has a peak impedance of 16 ohms into 4KHz the amp will clip at 50w, and if the lowest impedance occurs at 40Hz with 4ohms the amp will clip at 200w, but the frequency response at 50W will still be flat (within the amp's spec) across the entire spectrum up to 50w.
Trick question - what happens if the speaker is reproducing 4KHz and 40Hz *at the same time*?
Well, since one frequency is far higher than the other, there will be instants of time at which their peak amplitudes will simply sum together. If both frequency components have the same amplitude (which is highly unlikely with music), the amp could provide half of its maximum output voltage at each frequency, without clipping. Since for a given impedance power is proportional to volts squared, power delivery at each frequency at the clipping point would be 1/4 of the numbers you cited above: 12.5W at 4kHz, and 50W at 40Hz.
At least, I think so :-)
Regards,
-- Al