Is Pre-Amp needed

Never ever say never. It may be that you’ve not yet heard the right preamp for your system. By right I mean it makes the system sound better to your ears and by better I mean it adds the spice you didn’t know you’re missing.

OPPO degraded using a preamp? OPPO was designed as a bluray player with audio second in mind. Bluray players for music are a big nono IMO.

The preamp will always take the source and modify it according to it’s output. Remember, source is source, preamp and amp takes that source and runs it thru it’s circuitry.
Yes, end result changes. I’ve owned several OPPO’s, it was convenient for .flac files but I always ended up going back to the Simaudio i3 SE and Simaudio CD player for serious listening.
End result: bad recording, bad source does not get better, altho you can change it somewhat.

All volume controls add a sound . Its up to your ears as to what sounds good . Me , I have a Oppo 105 for my bluray and cable tv audio and video . I prefer a volume pot with the 105 , The 105 volume set to 100 % . 

@sthlm78 
 

Based on my limited experience, preamp is most important part after your speakers. 

 
I’d go: 
 
1) Speakers  
2) Placement/positioning 
3) Room treatment 
4) Power amp 
5) Pre-Amp / DAC

I believe the difference between those who like preamplification, vs. those who like passive, is as wide as analog vs digital, or tubes vs. ss. I have made my decision after owning some top tier preamps. My dacs drive my amplifiers beautifully, through passive units, and I simply hear more, of the recordings, then with top tier preamps in the systems. Not much more to talk about.