Ignore the 'nominal' ohm figure. There no such thing as 4 ohm speakers because impedance changes depending on frequency. Most good amps are designed to play 4 to 16 ohms.
They're easy to drive compared to some other speakers advertised as 6 or 8 ohms nominal.
My Dali opticons are advertised as 4 ohm nominal but they don't drop below 4 ohms. They are 4 to 11.5 depending on the frequency. The 11.5ohms is around 70 hz.
Is nominal another word for average? I read somewhere that Dali rubicons dont dip below 4. I just plugged my dali ceiling speakers into the integrated amp.
Blue Monday by new order sounded crap, but George Benson sounded pretty good. The sound was similar to the Bose acoustic wave jobbie in the kitchen, but the Bose had better bass.
The nominal is advertised by the manufacturer, some manufacturers will advertise their speakers as 8 ohm nominal as a marketing ploy when 6 ohms nominal is more accurate. Some also give a minimum and some will give a range (eg Canton)
There is a standard but manufacturers can advertise their speakers as anything they like. To be 8 ohm nominal, it means for a proportion of the time the impedance must be at least 8 ohms.
i would take ohms and rated watts with a pinch of salt. They're tested in house. Just like a car manufacturer's fuel consumption rated figures, they're tested in house.