I’ve spent a few weeks now experimenting with the Sprout connected to my Mac Air via USB using Bitperfect software (and iTunes). From my subjective point of view, disabling Bitperfect upsampling sounds best with the Wolfson DAC that Sprout has. I tried using each SRC algorithm Bitperfect supports along with enabling/disabling the “powers of 2/4” feature. Particularly with native 16/44.1, sending the stream at 24/44.1 (rather than 176.4 or 192) seems to sound best. Note that I haven’t found this to be the case with other DACs (or other DAC / clock implementations).
I invite others to experiment and share their impressions (with the same or other streaming software, naturally).