You can adjust the impedence of the Yamaha AVR to the impedence of your Pioneer front speakers. You do it when you calibrate the speakers using Yamaha's YPAO. Every manufacturer would not recommend mixing their equipment with others due to marketing/financial reasons but logic says you can (ie you take care of the impedence setting). What if the speaker manufacturer does not manufacture AVR and vice versa? When you buy the AVR and speakers of the same brand,  you probably get "better" sound (very subjective if not insignificant in reality) 'cos they were probably tested together during the development. I think all you need to get is an active subwoofer instead of the whole 5.1 speaker set.