I dont think these Yamaha's have bass management.I am leaning towards A-S701, however looks like Yamaha limits subwoofer cross over to 90Hz, while KEF LS 50 specs state a 79Hz crossover. Is there any workaround with this limitation?
Perhaps a low-pass built into the sub-out?, but no available high-pass for the speakers.
Consider the A-S501 + MiniDSP2x4HD(w/mic) comes in under you budget @$754 and gives you infinite options to integrate the sub & speakers and get them dialed in with your room.