Using an Oppo directly connected to Bryston amplifier can have high fidelity. However, this combination could also sound a little sterile in a way. Bryston amplifiers are usually very exacting and dynamic, but sometimes lack "life" - this could come across as being somewhat flat or sterile or non-engaging. It depends on what the listener wants. A preamp is used to help shape the sound. The AR SP6 preamp could definitely add some life and texture to the sound. You probably want to replace the tubes if they have not been touched since the 80's.
Should I eliminate my preamp?
I have been using my Audio Research tube preamp and Bryston amp to drive Magnepan speakers for years. Recently I added a Oppo blue ray player to my system and connected directly to my amp using the balance cables. The reason was to eliminate the signal having to go through another piece of equipment before it hits the amp. Am I wrong or what am I missing?