The DAC in your Sonos is being used at present. If you switch to the digital outs, then that digital will need to be converted to analog using an outboard DAC. DACs are more than simply chips as it relates to sound quality. There are dozens of factors including the quality of the power supply, implementation of digital stage, implementation of the analog stage, etc. The better your speakers and pre/power amp is, the more you will benefit from a better DAC.
HT: Panasonic BDT 460 | JVC RS600 | 106" x 45" Stewart ST 100 | Marantz AV7005 | NAD M27 | Elac Uni-Fi UF5, UC5, Debut B6 | Focal CMS Sub
Stereo: Cary Audio 306 SACD Pro | Cary Audio SLP-05 tube line level | Cary Audio SA-200.2 | Focal Diablo Utopia III
Den: Panasonic BDT 220 | 60" Panasonic VT60