most people tend to stick with the same impedance for all of their speakers, as it puts less strain on the amp. the 7100 is a particular amp that does not struggle with different impedances from the speakers. however if the speakers have different impedances they are probably different brands; mixing brands of speakers is not usually advised either as echoing can occur due to the nature of different tweeters being in the speakers. so if you are going to mix brands be sure to listen and make sure that they sound good together. the sub, being low frequency, can be a different brand w/o any side effects. as far as efficiency/sensitivity, higher dbs are more efficient and therefore require less power to produce the same spl. that is taking into account that they are both rated exactly the same, and if they are different brands they most probably are not. with a pre/pro that has channel gains, etc. its not a big issue because you can get equal sound out of your speakers by making adjustments to the gains. youre not going to find the answers to all your audio questions in a forum though. sound needs to be heard. hopefully this reaffirmed/cleared up the previous posts and you understand a little more. im sure soundhound will disagree with me as he is prone to do. :-)