There was something going on with that and it was subtle. However the center channel didn't seem similarly affected. Is the center from the Studio 100 series? ... if that is in fact how it works. I know it was from a model up or more current than the Monitor 7 mains.
I wonder how the vocals sound through the center being driven by the RX-V2600. I know the Studio 100 mains are difficult to drive. Greg and Warp both swore by externally amplifying them. Is there a published impedance/phase curve for your center.
That is hugely disappointing about the A500's. The thing is that not all of them were problematic. I wonder if there is a known fix to the known/suspected distortion issue. I'd hate to think that you now have 5 bricks.