EJR1953, On the Focal Sopra's, I just found graphs for you impedance and phase, the - phase angle is not too bad, but the real measured impedance is quite tough, from 80hz to 120hz it's 2-3ohms, so a good 4ohm solid state with good current ability is a good idea for these to get good controlled dynamic bass.
Last two graphs in this link.
http://www.soundstagenetwork.com/index.php?option=com_content&view=article&id=1445:nrc-measurements-focal-sopra-no2-loudspeakers&catid=77:loudspeaker-measurements&Itemid=153
Cheers George

