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...

@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