Volume control

I would guess your CD player’s fixed outputs are simply outputting more voltage than the OPPOs RCA jacks.   What is your preamp?  CD player?  Any chance the CD is balanced XLR connection vs. the OPPO RCA jacks?

What’s the amp’s sensitivity?

Which Oppo do you have? Does it have a "power on" setting for the variable volume? Perhaps your variable volume is set to a lower level (not full output).

Modern CD players have a max output of about 2 Vrms. Your Oppo probably also has a max output of about 2 Vrms. Theoretically they would be pretty close in volume. That is, unless your variable volume control on the Oppo is set a level less than max volume. Do you know if your Oppo has the volume setting pre-set at a certain level? If you set the variable volume on the Oppo to max, it should be relatively close to your CD player.

But! Be careful if you are connecting the Oppo directly to your power amp (bypassing the pre-amp). If you have the volume set to max output when you play a cd through the Oppo, and the volume is set to max, it is going to be VERY LOUD!

Thanks for the response. My preamp is Audio Research SP6 and my CD player is a Rotel. The amp is Bryston 4B st. I do not know the sensitivity of the amp. My Oppo is model 105.