Why not try using the Oppo 105 direct, using it’s own volume control, you may even find that you like it better than having any preamp in between. As it has low output impedance (which is perfect impedance match for any poweramp/s) and more than enough volts to drive any amp/s into clipping, and then you have the benefit of having no added colouration of more electronics in the signal path.
I just noticed that your Bryston has a switch so you can use it just as a poweramp input, have you ever tried the Oppo direct into this and used the Oppo’s volume control?
Cheers George