audphile1 Thanks for your suggestions. I did exactly how you describe it (no apples vs. oranges): the USB signal either directly from my Mac or from the EVO goes into the MSB USB/Optical converter module, and the fiber optic cable runs into my DAC (at the opposite end of the room). And yes, I am not using any DSP/upsampling from either source, just as you said. The MSB Discrete is a fabulous DAC using the proprietary R2R technology that makes this brand so outstanding in terms of musicality and yet precision (I used their flagship Premier Platinum Signature for years, but it would not handle DSD).
If you look at this thread, there are quite a few members who share my listening experience with ROON, albeit maybe not so pronouncedly. I am in communication with the gentleman in Belgium who gave us the EVO: he too feels that ROON might be not the last word in clarity; so he is writing new firmware for another very popular audiophile player as we speak. There might also be an EVO-specific sound at play: your Aurender server is well known for its clarity and transparency (the MSB brothers use it at their homes), while others call it too lean. The reviews of the EVO describe its sound as very "musical" and "comforting" (using the ROON player). There might be also a personal preference at play here.
In any case, Frederic (the designer of the EVO) graciously agreed to have a look at my unit paying particular attention at the USB output patine, which might be faulty. We will see.
As to my initial question: the members who say that there is no such thing as a ROON sound ("bits are bits") are on one side, while others say that the more CPU engagement a player program needs, the more the sound suffers. I am a novice when it comes to computer audio, and hence cannot say who is right or wrong. Moreover, I might indeed not have completely optimized my signal path yet. As for now: vinyl still rules IMHO.
[MacBook Air with M1 chip; 432EVO Aeon server with shielded Ethernet connection directly to a Comcast router; ROON via ApplePlay (FLAC), or Apple Music (ALAC), or Audirvana, or HQPlayer (for DSD as DoP); MSB USB/fiber-optic connection; MSB Discrete DAC; Supratek Grange preamp; Linkwitz Labs Powerbox (x2) active crossover and N2Core amplification (x 10 total); Linkwitz LX521 active channel (5x2) open baffle speakers; all non-digital cables and connectors home-made 9N silver solid core].