Regarding the M5 impedance from Ian:

"To stay within the technical definition for nominal impedance, it is a 6.25 ohm speaker but since the 5 ohm low point is at a higher frequency than the normal, which is normally below 100 Hz, I went with 8 ohms. From a practical point of view we are mainly concerned with if it will operate safely attached to an 8 ohm receiver. We got away from the old 6 ohm ratings we used to have on some speakers as it was proving just too confusing."

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated