That's a logical answer Serg. So an amplifier with a 6 Ohm rating means it is suitable for an 8 Ohm nominal speaker. I am sure the lawyers would not allow the term "nominal" in the spec. You can get away with it as so many people think they need 200W when they are using all of 20.
The one disaster I remember was I think, an Apogee ribbon where the load was almost capacitive. It would upset almost all amps at the time. Shame, nice speakers.
For those who understand how transistors work, really serious amplifiers have more pairs of outputs. A hint is the 4 Ohm power rating will be close to twice the 8 Ohm rating. That means the power supply has enough current to maintain the rail voltage and it has enough outputs to distribute the load. Very few go to this expense, but sonically it is worth it. Take a basic Self Blameless and double the outputs to see/hear what I mean. I did it on a cheap Chinese MV-50 clone kit. Real benefit is a more linear operating range of the outputs. Marketing department would then say to up the voltage and advertise it as a much bigger amp as that is what sells and you are back to square one.
Beef I have with specs is class D is by standard agreement, rated at 1% distortion. So some nice new Hypex may be .001% @ 90W, they list it at 1% @ 100W. Fine for cars ( where the standard came from) but not for hi-fi. Fortunately a few respectable companies give us a graph. Unfortunately, some not so nice amps are .5% across the band!
Yes, if only loudspeaker manufacturers took any notice of the IEC standard. So many quote nominal 8 ohms, because that's what the public understand, it's a nice single number. Quoting impedance for example as nominal 8 ohms, minimum 3.8 ohms @ 80Hz would only confuse people.
As to the Apogee ribbon, that was a pretty much purely resistive load, not capacitative, but the Scintilla presented a load of only 1 ohms, which didn't half upset most amps. I understand that the Krell was developed as one of the very few amplifiers of the era capable of driving the Scintilla.
Doubling output into 4 ohms from 8 is indeed what a 'perfect' amplifier with a fully stabilised supply will do, and in practice there are very few Class A or AB amplifiers that actually do it. There's the output impedance of the amplifier itself, which won't be zero, although can be close, and the regulation of the power supply. One way of making sure it'll happen is to design the amplifier to provide, say, 200 watts into 4 ohms, (or 400 watts into 2 ohms!) then it'll provide 100 watts into 8 ohms easily. The temptation then is for commercial reasons to rate the amplifier at, say, 120 watts into 8 ohms, in which case it won't double power into 4 ohms, even though it's the same amplifier!
As to the 1% distortion figure, it's a way of getting the power number up, without being silly about distortion, in that very brief clipping that takes the amp to 1% will be generally inaudible provided the amp clips cleanly and recovers fast. I measure my amps to the onset of clipping, so still at 0.01% or whatever. One caveat is that valve / tube amps may not have a fixed sharp clipping point, so rating those at 1% distortion makes a certain sense.
S.