I heard those side by side about a year and a half ago driving Raidho XT3's. As I recall they sounded very close with the slight edge going to the U300. The U300 had a hair more control over the music IMO. We did also try with and without SAM and I preferred no SAM. But also if I remember correctly the SAM was for the X3 not the XT3.
That said I do own the U300 driving D2.1's
That said I do own the U300 driving D2.1's