It's either placebo or there is something different in the chain effecting the sound. There is no reason whatsoever that the dongle and RME should sound any different. Amps and DACs simply don't effect soundstage, timbre, etc unless there's something esoteric or weird in the chain or it's a tube amp, but even then the differences are extremely minor.
Did you make sure they were precisely volume matched and have you done blind A/B testing (after removing any variables between the two setups)?