We tried the Topping D90 against the SMSL M400 both in the same price range and preferred and bought the SMSL M400. The Topping was a bit forward and bright for our taste, while the SMSL was just right. Not as good as my Chord Hugo TT2 but much closer than I had ever expected for seven times less money. Best DAC I've ever heard under $1000 although I'd admit I've not heard the R2R examples.
Happy listening!