Yamaha RX-A850 with B&K 200.7 Hum

Was trying to take an economical approach to updating my B&K Ref50 and chose the Yamaha RX-A850 to pair with my 200.7. It only has the unbalanced pre-out connections and there is the dreaded hum. I didn't notice any hum with the Ref50 but I was using the balanced connections. Has anyone had a similar experience? Really don't want to spend $2k on another pre-pro. Any suggestion? Thanks.