If you're seeing equal spl reduction across all channels, this doesn't sound like it could be the amp. The 5 channels are all separate, with their own power supplies. Are you using the original power cord for the amp?
I noticed there's a recent firmware update for the Yamaha. Stated something about ypao volume function.http://www.hardwarezone.com.sg/tech-news-new-firmware-updates-bring-4k60p-support-several-receivers-yamahas-aventage-a30-series