My understanding is that the Oppo volume control is only active when using the “analog” outputs. The digital (coax/Toslink) would not work to control this.
I thought the Ayre has the ability to act as a preamp- if this is true you should be able to attenuate volume from the Ayre directly to control the S300.
I would still go into the Oppo’s internal setup menu and select speaker size, any DSD/PCM priority and set the volume output to 100% anyway. Go coax into Ayre from the Oppo.