Al,
I cross-posted with you, so I hadn't read your post, before I posted.
Are you saying that an erratic electrical speaker impedance curve doesn't actually present a more difficult load to the amplifier in this case, because it's still within the acceptable limits of the amplifiers current delivering ability, and there won't be a problem with a frequency db imbalance, unless the curve dips far enough below the amplifiers minimal impedance, lets say 2 ohms?
Rich
Hi Rich,
That is sort of what I am saying, but I would put it a little differently. As long as the output impedance of the amplifier is small compared to the impedance of the speaker at the lowest point of its impedance vs. frequency curve, and as long as the amplifier is not called upon to deliver more current than it is capable of delivering, then a tonal imbalance will not result.
The fact that an amplifier can double its output power into 4 ohms, compared to 8 ohms, is an indication that it has good output current capability. In this case, the fact that the amplifier can put 250W into 8 ohms but only 400W into 4 ohms, is an indication that its output is being limited into 4 ohms by its output current capability, and therefore the peak volume levels it can produce accurately into a speaker which dips to 4 ohms will be less than for a more benign speaker load (everything else being equal). But as long as the output current capability of the amplifier is not exceeded (and keep in mind that we are presumably talking here about tonal balance at moderate volumes), and as long as the output impedance of the amplifier is low, no frequency response imbalance will result.
A notable exception to all of this would be amplifiers which have tube output stages and are output-transformerless, such as the Atmasphere's, which may have an output impedance in the vicinity of 4 ohms or more. That would definitely result in a tonal imbalance working into a speaker that is basically 4 ohms in the bass and 8 ohms in the treble (see Stereophile's impedance measurements on the Aerial 7B). I recall Tvad once mentioning that he had an Atmasphere at one point, and perhaps that is what prompted his comment.
Regards,
-- Al