Question on Impedance-

[Dieselboy]

I'm looking a set of speakers that list the nominal impedance as 6 ohm and the minimum as 4 ohm. These are rear surround speakers. If I use them in conjuction with my other current 8 ohm speakers (R/L/C) will this cause issues? My reciever is an Elite -21, I'm sort of a noob when it comes to this subject matter, I know that the lower the number, the harder your reciever/amp has to work, but beyond that I don't know too much more. So what's the difference between the nominal and the minimum #'s? Somebody please educate me as to the in's and out's of this subject and whether or not these speakers are a good idea for my 5.1 system?

Thanks!