4ohm vs 8 ohm......why one or the other.......

The early audio designer had to settle for some figures to work with I guess.
Did it date back to the little note left for the milkman..?
2 pints to a quart, 4 quarts to a gallon...8 pints for an audiophiles' milkshake party?

There are quite a few well-known combinations of low-impedance speakers that mate very well with tube equipment. Magnepan and MartinLogan speakers both present pretty low impedence loads and both are generally said to sound better with tube equipment. I know my ML's sound even nicer with tube euqipment than with solid state.

.....Garfish, some speaker manufacturers (Thiel?) intentionaly correct their crossovers so that the speakers present a fairly consistant load. I believe the intent is to offer amplitude coherence and to help keep the amplifier stable by limiting the electronic gymnastics. Though some claim that these measures suck the life from the over all sound, it seems like a "sound" idea to me.
.....Philojet, high impedance loads can be just as difficult for solid state amps as low impedance loads are to tube amps. In as much as solid state amps tend to put out more power into low impedance loads, the inverse is true as well, solid state amps put out less power into high impedance loads. Most people don't realize that even a mighty Krell with will wince faced with a 32 Ohm load as much as the average (there are always exceptions) tube amp will wince when faced with a 1 Ohm load. Solid state amps don't have as much a problem today, as in yesteryear, when there were many more speakers with loads over 16 Ohms. This is not to say one is better than the other. It's about using the right tool for the job at hand.

typically a speaker with a higher impedance is more desired than one with a low impedance, especially if you have a tube amp - which will sound like it has tighter bass if your speaker has a 12 OHM load instead of a 4 OHM load for example.

A tube amp does not change power output when speaker impedance changes like a solid state amp does, when you lower impedance into a solid state amp they put out more power but a tube amp usually puts out about the same amount of power when the impedance changes.

Usually speaker impedance is more important when you have a tube amp.

This is a very rough answer and only a guideline as there are exceptions - and certainly better answers - I am sure.

The impedence of speakers is the resistance to being driven and is measured in Ohms, and the resistance, or load is not constant over the frequency range. So, while my Vandersteen speakers have a NOMINAL impedence rating of 6 Ohms, the Vandersteen manual states that the impedence drops to a minimum of 4 Ohms, and the driving amplifier should be stable into a 4 Ohm load. Typical speaker loads may range in impedence from say 2 Ohms to 20 Ohms or more.

Each time speaker load is halved, current need is doubled, thus Magnepan speakers that have an inherently low nominal impedence-- I think about 2.8 Ohms-- require an amp with high current output, and such low impedence speakers should be driven by amps that are stable into 2 Ohms (or lower). A good high current amp will double it's output in watts as the impedence (load) is halved.

As to your actual question, I don't think that GOOD speaker designers aim for a specific resistance in Ohms, rather their MAIN goal is making a very musical sounding speaker, and with some designs they may end up with a speaker that requires high current. As noted, Magnepan (planar design) speakers are notoriously difficult to drive, but they just need a good high current amp. Thiel dynamic speakers, I believe, typically present a below 4 Ohm load also.

It seems that most "consumer grade" speakers are designed specifically with a nominal 8 Ohm load (or above), and thus almost all "consumer receivers" (Sony, Pioneer, Kenwood and others) are only rated into an 8 Ohm load. An inadequately powered amplifier can ruin a speaker if it is unable to provide the power the speaker needs. Cheers. Craig