I have a pair of Monitor 5Bs that I purchased new in late 1989. I just noticed that the owner's manual states the nominal impedance as 4 ohms. This seems really low for a $225 speaker. The same manual shows all of the smaller Monitors as well as the 7C also at 4 ohms. The 10B and 11 have 6 ohm ratings. Is this correct? If it is, there's some pretty expensive (to me) equipment out there that can't drive these things.