Saki70, thanks for your comments.
What we are talking about is 2 things: 1) output impedance as an actual raw impedance, not complicated by negative feedback (IOW 'open loop') and 2) servo gain- the amount of negative loop feedback employed.
A lot of designers see these two as the same, but they are not. A variety of Voltage Paradigm speakers *require* that the amp have some sort of feedback to accommodate the otherwise improbable impedance curves that have resulted. The feedback is part of mechanism that allows the amplifier to accommodate peaks as well as dips in the curve. You can do this with raw impedance alone, but feedback makes it easier- your amp does not have to have such a low open loop impedance.
With higher impedance amplifiers, in order to get flat frequency response on such speakers, the role of feedback becomes more prodigious. However, many of these amplifiers are probably tubes, and often tube designers will eschew large amounts of feedback as the amplifier will often exhibit some linearity without, something that is rare in the transistor world. Their hope is that you will do the right thing and use these amps on a speaker that has a higher impedance. FWIW the thinking here revolves entirely around sonic performance rather than the ram ability to simply drive a low impedance, something that usually has little to do with overall sound quality.
So we are talking about a spectrum- as output impedance is increased and servo gain decreased, the voltage/current ratio that describes the output of the amplifier changes with it. So there is not a hard and fast rule.
In the past I've seen a lot of DIY hobbyists try to add loop feedback to a 'current source' amplifier with the hopes of getting it to play a four ohm load better- with more power. It does not work. That is because the open loop impedance of the amplifier is too high to be adequate for four ohms. You can reduce distortion and flatten the frequency response using servo gain, but you can't change the power. That is why I say that open loop output impedance and servo gain are different phenomena.
The pity of this whole thing is the idea that the ability to drive 4 ohm loads is a sign of being 'beefy' or 'gutsy' (somehow better anyway) in the amp. The fact of the matter is no transistor amplifier sounds right on 4 ohms, nor does any tube amp. If you want to really see what either one is really capable of, you need a higher impedance -16 ohms is nice- to do that.
At higher impedances speaker cables are far less critical in the overall sound and all amplifiers will exhibit less colorations due to reduced distortion. Transistor coloration BTW is the harshness caused by odd-ordered harmonics at very low levels. Tube coloration is the added 'warmth' or 'bloom' that is a product of even-ordered harmonics that are at a more pronounced level. So transistors will sound smoother with more detail and tubes will sound more neutral with more detail. Win win.
Sorry there was not a simple answer to your question!
