Is Pre-Amp needed

A preamp can never make the sound more accurate, it can only degrade (it has its own frequency response, THD, etc.), though the degradation is usually not much of an issue for any good one.

Now, volume control and voltage gain are what many pre-amps can provide that many sources need.

However, the Oppo has been measured and shows its digital linearity (for volume control) is excellent, so no worry about worsening S/N as you decrease the volume; it also has more than enough voltage.

https://www.audiosciencereview.com/forum/index.php?threads/review-and-measurements-of-oppo-udp-205-u...

A preamp can never make the sound more accurate, it can only degrade

This statement is 100% correct. Going direct, has less distortion and colouration, is more transparent and dynamic.
An added preamp preamp, if prefered can only add colourations/distortions as a "band aid fix" for a "source sound" they don’t like.
Get a better source, instead of putting a very expensive bandaid on it. "The best preamp is no preamp."

Cheers George

Re the accuracy debate: interesting comment in Stereophile from one of the writers. He quoted JGH as having said something along the lines of...Accuracy is something all audiophiles say they want, but when they get it they don't like it.

So...maybe using a preamp is less accurate, but it sure sounds better to me.

I will never use a preamp again.