In reply to:
But I also believe that as volume levels increase, those dummy speaker results will get exacerbated. Why? Because that impedance linearity fades away under higher power. I own "the most powerful receiver Harman Kardon made in 30 years," the HK 3470. It puts out 100wpc into 8 ohms, but something like 130wpc into 4 ohms. A really good 100wpc amp would actually put out almost 200wpc into 4 ohms. The discrepency means that my amp gets progressively less linear, with respect to impedance, as power output increases.
Cooper,
We need to separate two distinct issues here: (1) impedance linearity of maximum available power, and (2) power-level dependency of frequency responses under realistic loads. We should not mix these up together.
(1) Yes, most moderately-priced receivers/amps cannot provide a linear increase of maximum power when the load impedance is reduced. This is simply because there are limits in maximum current draw in these amps, as well as limits in power-rail voltage regulation under high-current situations. Also, as you said, there are definitive differences even among the amps/receivers with similar maximum stationary power outputs as to how well they handle transient peak power demands (so-called power researve or "crest factor"). These differences are certainly measurable, and can be audible when one tries to drive an insensitive/inefficient pair of speakers (such as the Magnepans) in a very voluminous room. But when you are driving speakers of decent sensitivity (like the Axioms) at moderate volume levels in a moderate-sized room, the amp will hardly put out more than a few 10's of watts in 99.9% of music passages. My point is, these max power-related differences CANNOT render the "sonic character" of the amp, which supposedly is present constantly through the music.
(2) These limitations in current draw and power-supply regulations seen in many moderately priced amps DO NOT result in a modulation of the frequency response until the actual power output approaches the maximum available power. For example, the frequency response of your H/K 3470 under a realistic speaker load will be virtually flat (except for the tiny dips due to the finite output impedance as explained earlier) when driven at 1W, 10W or 100W into 4 ohms (until it approaches the 130W limit). The frequency response DOES NOT dip more when it is putting out 100W, as compared to the response at 1W.
So, I do agree with you that the dynamic power handling measurably differ from amp to amp, including the amount of transient power reserve as compared with the amp's sustained maximum power, and the gracefulness of distortions when the maximum power is reached. These attribute can be an important factor under certain situations where you do actually encounter a three-digit power demand frequently enough so that the difference is audible.
However, my point remains that these attributes related to the maximum available power output CANNOT explain the alleged "sonic signature" of the amps/receivers.
