The B&K 507 equals quality
The Yamaha z9 runs pretty hot. If you had any other run of the mill receiver I'd say make the jump to the Yamaha.
However, if I was looking across may living room at the B&K-507, I think I'd stick with the 507. The solid sound and build quality of the B&K is hard to beat except by hi-end separates.
Don't be fooled by the Yamaha's whistles, bells, and trinkets. Sound is where it's at.
By the way I own a Yammy RX-2600 that I'm getting ready to sell and buy a B&K