I use a Twisted Pear Buffalo2 dac with my Sonos. The ESS Sabre32 chip does internal reclocking and fixes a lot of the jitter problems that come out of the Sonos. But adding upgrade digital SPDIF cable and upgrade spdif RCA jack both made big improvements and there is still more things to try.
The Sonos is consumer quality electronics. You buy it for the functionality, then you apply aftermarket tweaks to bring the SQ up to snuff. The functionality is worth it, to me.
I thought about modding the Sonos' guts. There are some obvious things to do in there, but after it was all done, it is still consumer design in a tiny box and there will be unsolvable problems. So applying a scrubber like Steve's, or using a modern jitter rejecting DAC is still necessary anyway.