I agree with Bomarc's comment's above. The nominal impedance rating is not regulated by the Bureaucrats in D.C. and is little more than a marketing tool. It is much more useful to consider the low DCR point to determine the affective load on a solid state amplifier. The swing of the impedance curve is almost completely insignificant when using a solid state amplifier.
When considering a tube amp the swing of the impedance curve becomes more significant. Sean's remarks above are quite accurate. He expressed this issue much better than I could. The nominal impedance was important when speakers were built with something that actually resembled a nominal load. Speaker builders generally don't pay much attention to the tube guys because they represent a very small part of the market. Hence the impededance swings on a typical speaker are huge. This will directly affect the response curve when a tube amp is used. This seems to be more significant with the SET variety and less significant with the Push-Pull variety. I believe it has something to to with the dampening factor.
Dave
When considering a tube amp the swing of the impedance curve becomes more significant. Sean's remarks above are quite accurate. He expressed this issue much better than I could. The nominal impedance was important when speakers were built with something that actually resembled a nominal load. Speaker builders generally don't pay much attention to the tube guys because they represent a very small part of the market. Hence the impededance swings on a typical speaker are huge. This will directly affect the response curve when a tube amp is used. This seems to be more significant with the SET variety and less significant with the Push-Pull variety. I believe it has something to to with the dampening factor.
Dave