This is one hypothesis, and an appealing one. But all modern preamps, be they tube or solid state, measure vanishingly low in distortion of any kind. And people report improvements using both tube and solid state preamps over running the DSD directly into an amp. Euphonic distortion does not appear to be the source of the improvement.
An alternate hypothesis, which also explains why whether adding a preamp improves the sound in a given system, is improved gain staging. I have found good gain staging makes an audible improvident in the studio. I expect this to be the same in home audio.