Is Pre-Amp needed

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

I will never use a preamp again. 

Based on my limited experience, preamp is most important part after your speakers. You can read all the numbers and statements that logically sounds correct but if you add a good preamplifier, you understand why people use preamps.
p.s)Some DACs equipped with a in built preamp but still I prefer a separate preamp specifically a tube preamp!

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.