There has been the question of why there is 4 ohms. The answer is that it makes it easier to make power with transistor amps.
But making power is not by any stretch the same as making the best sound for a given amp. You can look at the specs of any amplifier made and one thing is made clear over and over- with lower impedances the amp will make more distortion.
There is a device called the ZERO which allows you to drive a 4 ohm load, while the amp is loaded at 16 ohms. Even people with transistor amps and class D have reported that the the system sounds better when using the ZEROs rather than driving directly, even if the amp has no apparent problem driving the lower impedance.
The reason why 4 ohms is problematic is several. For conventional transistors, there is a non-linear capacitance exists as part of the junction of nearly any semiconductor. The capacitance is magnified by increased current through the device. This causes increased odd-ordered harmonics; IOW if you put the amp on a high impedance load, it will have less of these odd orders and so will sound smoother.
Output transformers of conventional tube amps will loose as much as an octave of low frequency bandwidth when driving 4 ohms as opposed to 8 or 16 ohms. The transformer will also be less efficient and heat up more and there will be higher distortion (open loop).
Finally, speaker cables become critical- you should not expect to run any speaker cable over about 2 meters with 4 ohms speakers if you want the best response. In addition, the gauge becomes important, a few hundredths of an ohm can have surprising effects on the damping factor.
Four ohms became more popular when the transistor made its way into hifi. It is a way of possibly getting more power out of the amp. Only a few years earlier when tubes were the only game in town, speaker efficiency was pretty important (which is why there were so many horn systems from the 50s and 60s). Transistor power was cheaper to make, and speaker manufacturers realized that if they made the speakers less efficient, they were a lot cheaper to make too. Four ohms was a way of getting back some of the 'efficiency' but of course what we are really talking about is 'sensitivity'! If you don't know the difference, go back and look at the link I initially posted in this thread.
IOW follow the dollars- its cheaper to make less efficient speakers so four ohms 'helps' with 'drivability'.
But in the world of high end audio there really is no argument for lower impedance, as all amplifiers known do not sound as good on the lower impedances, increased power or not. In high end audio, its about sound quality as opposed to sound pressure, so the additional 3 db you get by doubling power from 8 ohms down to 4 isn't a big deal.
Now there are a lot of software programs for designing speakers that say if you put two drivers in parallel, you double the efficiency (which is part of why you see such loads show up so often). This is incorrect- you double the *sensitivity*. IOW add 3 db but that 3 db comes from the transistor amp, not the speaker!
There is also the myth of 'control' that the amp has over the speaker. Any amplifier will have more control over 2 drivers in series than in parallel, yet you will see many who insist its the other way around. But if you look at it from a damping factor perspective, an amp that drives a 4 ohm load with a 20:1 damping factor will have an 80:1 damping factor on the same drivers in series...
