The Benchmark is a pro unit with exceedingly low distortion and is generally darn near flawless; it's about as close to the proverbial "straight wire" as any gear can come. I seriously doubt that it is degrading the sound. I also doubt that the other units, even if they don't measure as well, are coloring the sound either.
So, sorry, but I tend to agree with Mtry. Small differences in level (down to 0.1dB!) can play tricks on you. That's a fact well established by research. But if you still don't like it, I wouldn't mind having a Benchmark.