Most high-quality loudspeakers are 4-Ohms

>>12-02-09: Unsound
Most ss amps aren't at their best into 16 Ohms.<<

That might not be the case.

I seem to recall in a previous thread Ralph Karsten saying that all amps, power output notwithstanding, perform better with higher impedance loads.

Maybe Ralph can jump in here and clarify.

Bill, I suspect that at higher impedances ss amps might be able to sustain a higher bias longer, but at the cost of reduced ultimate power. I suspect that the speakers sensitivity coupled with room size and ultimate volume sought would of have to be added to the equation. Still, I think 16 Ohms in most cases would not be the best load for most ss amps.

A solid state amp will put out less power maximum power into a 16 ohm load, which of course is an issue if the amp may be pushed into clipping. But if the amp will produce adequate headroom into a high impedance load, in my opinion there's a valid argument in favor of relatively high impedance speakers.

If you look at the distortion curves published by Stereophile at 8, 4 and 2 ohms, you will see that below the onset of clipping the distortion is usually lower into a high impedance load. Due to a psychoacoustic phenomenon called "masking" the distortion at low power levels is of considerably greater subjective consequence than distortion at high power levels, so you want to focus on the low-power end of the distortion curves (again assuming you're not driving the amp into clipping). Personally I'd like to see what's happening down at the milliwatt level but that information isn't given.

Let me just comment that THD measurements are not reliable indicators of relative sound quality from one amp to another, as design choices that minimize THD are often counter-productive from a subjective standpoint (small amounts of high, odd-order distortion are far more audible and objectionable to the ear than are very large amounts of second harmonic distortion; in other words, the industry is meauring with the wrong yardstick). But THD measurements made on the same amplifier under different load conditions are reliable indicators of relative sound quality because they tell us how close to ideal that amp's performance is into that load.

4 ohm nominal designs which often dip down to 1.5 ohms will provide significant challenges for most power amplifiers. They will also cause speaker cables to start to make an audible difference and of course the heat generated in these type voice coils will result in significant thermal compression also. (This can make cheap speakers more reliable as the thermal compression acts as protection for the drivers when driven hard and make them sound terrible so you turn the volume down)

4 ohms are best avoided if you are at all serious about accurate audio reproduction.

8 ohm or 16 ohm works fine... most tweeters are around 20 ohms.

And yet some of the speakers that have demonstrated some of the best testing results have low impedances.