power/impedence reciprocity

Sugar, cost is not a factor here. Look at what the amps clip at and then look at what they are rated for at the various impedances involved. The closer that they come to "doubling down" at the point of clipping, the truer the statement "high current" and "true voltage source" would apply.

The fact that the 100 wpc integrated that you mention does not go up drastically in power as impedance is halved tells me that this manufacturer is actually giving you "more legit" power ratings on this model. He could have rated the 100 wpc model at 70 wpc at 8 and then 140 wpc at 4 and marketed it as "high current", "high dynamic headroom", etc.. As i mentioned before, he would have simply de-rated the high impedance power to make the low impedance output look even more impressive. It would still be the same amp, just with different specs. This follows suite regardless of price.

As to the 50 wpc MORE than doubling as impedance is halved, i would surely tend to believe that the 8 ohm output level has been "deflated". I would venture to say that it is probably quite a bit more powerful than 50 wpc at 8 ohms. Only testing and measurements of the actual component would verify this though.

Some amps just come closer to REALLY doubling down than others. The power output at clipping tells you the amps REAL capacity and just how "beefy" the power supply and output section really are. Sean
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That why I mentioned cost. Generally speaking many better made (higher cost) amps come closer to doubling.

This topic prompts me to finally ask a question that's been bubbling under my hat for years. Why is it, factors of preference aside, that both a SS amp that comes close to "doubling down" and a tube amp that doesn't can basically get the same job done in most cases? Average speakers, even if they don't drop to very low impedance levels, still present a load that flucuates markedly with frequency. Why don't tube amps as a catagory seem to suffer some pronounced sonic deficiency compared to "voltage source" SS types, as long as the minimum doesn't drop too far below the output tap rating? Could the ability to increase output as impedance drops not really be that important? Based on the "doubling" propaganda you read/hear, someone who had never heard a tube amp before would have to be excused for thinking that the breed must deviate noticeably from amplitudinal fidelity to the imput based on load impedance vs. frequency variations. On the Atma-Sphere website, Ralph Karsten argues in a white paper that what's actually desirable is for an amp to be able to maintain a *constant* output (watts) as frequency/impedance changes. His rationale is based on the inverse of the conventional wisdom - he criticizes SS amps for *dropping* in output power as impedance *increases*. (This, of course, is something his OTL tube amps supposedly don't exhibit.) But my own ears tell me that, within reason (and putting aside separate issues such as power vs. sensitivity, or the effects of output impedance on damping and frequency response), both tube and SS amps can drive most speakers possessed of average impedance curves and yield essentially comparable sound, despite the rhetoric. What gives?