I do not have a dog in this hunt, as I sold my DS Sr. and Matrix some months ago, but for the record, running a Matrix between an Auralic Aries G2 and a DS Sr. made a meaningful audible improvement in my system. Also, the person to whom I sold my Matrix runs it between an Aries G2 and a DS Sr. (FWIW, his speakers retail for roughly 30x the price of mine.)
If we start with the notion that the DS’s I2S input sounds better than its USB input, then it tends to follow that the Matrix should improve the sound of all USB-based sources. (Or, put differently, the Matrix arguably sounds better converting USB to I2S externally than the DS sounds doing the conversion internally.) Please don’t take my word for it, though. See Posts 1375-76, above.