I have two RME ADI-2 DAC FS units. I ran one with the Pre90 and the other alone. I used them on two BMR monitor speaker setups with a MacBook Pro feeding each RME via Audirvana. All I do is mute one system and unmute the other. Same sound level. You can easily tell the unit without the Pre90 is crystal clear while the RME with the Pre90 is significantly coloring the sound. They don't sound anything alike. It's an easy test. Just mute/unmute each setup.
Of course it begs the question whether that is something that would hold up to double blind testing. Not doubting your perception, but also confused as a transparent device should be well, transparent.Is it possible one of the RME's is out of spec?--you could try switching the feed to the Topping from one RME to the other. Groping for explanations here.
And here's our standard ASR answer set - blind testing needed and something's "broken".
Of course, we shouldn't doubt perception, and we can go further to the point that the Topping doesn't fit
@amper42's use case. But can we even assume that the RME is the "crystal clear" device in this situation? All we have is a fairly crude description of a subjective difference.
At least with a Stereophile review, we have disclosure of the full system in use, and the language is usually chosen to represent that the reviewer is giving a subjective review or comparison in a specific setup. I can (if I can be bothered) check out each other item in the system - they are often reviewed already - and look at impedence matching of components, calculate system gain and power, see that the amplifier can drive the speakers, etc.etc.
I would hope that paid reviewers actually do all of that work anyway, even if such minutae is way too boring to be in a published review.
We don't know enough about the setup
@amper42 is using, or the test protocol, to go beyond it not working in that particular setup (so don't use it there).