BW, the 671 would probably do the job if, as always is pointed out, they're not played so loud and so long that overheating shuts down the receiver's protective circuits. Having said that, for reasons mentioned here before, in the price range you're apparently considering(about $500), I'd suggest you look instead at the Onkyo 709 or Denon 2312 from Accessories4less. Maybe even a better suggestion would be to forget the 978 and get a receiver at this time such as the Onkyo 3009 or Denon 4311 would should meet your needs in total.
As Alan pointed out, the lower impedance setting should never be used, regardless of what the rated impedance of the speakers is. This is a legally required provision to help prevent overheating and burning down the city, but does this by limiting the voltage available from the receiver's power supply section. The result is to limit power(the maximum is cut roughly in half)and maximum performance.
