The output voltage of the AR SP6 is higher than the Oppo's. If the input sensitivity of the Bryston 4B ST is similar to the Bryston 4B's you should be able to achieve maximum power output with either unit preceding it before having the volume control at the maximum setting.
Remember the volume control acts more like a brake than an accelerator. In this case the lower output voltage will allow greater range and perhaps more consistently precise volume control settings.