Should I eliminate my preamp?

My preamp is AR SP6 b (circa 1980).

I thought using the volume control on a digital device such as the OPs UDP would degrade sound from truncating bits at lower volume settings, hence the need for a pre-amp with (presumably) quality volume control in the analog domain.

The argument about dynamics is wrong. An electrical signal comes out of the Oppo and goes into the power amp. What would be pre amp do? Change the signal?
As for digital volume control. That argument was true years ago. These days DACs have enough bit depth to avoid this problem. There is a paper by ESS somewhere but I do not have the time now to look for it.

You've done it so do you prefer the sound direct vs your preamp?  Your preamp being some 38 years old may be in need of a refresh and certainly a retube as suggested above.

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.