Well, for what it's worth, I'm currently using a much cheaper outboard DAC option in lieu of the RR 2150's USB input, but it's clearly an improvement.

Back in January, someone in a Stereophile forum suggested I try an M-Audio Fast Track Pro, a pro-audio recorder interface designed for musicians that has a USB input. Lists for $200, but I found an open-box new one for about $130. The RR 2150 is my main/only source for 2-channel audio, so ultimately I couldn't see spending as much on an outboard DAC as I did on my receiver––though I definitely considered it. Nevertheless, the music on my MacBook has been bumped up a notch.