I've used a Sonos ZP90 connected through an external dac for a couple of years now. My observations on this in terms of 'high end sound' are as follows:
- unfortunately, 'digital out' from the ZP90 doesn't necessarily equate to 'job done' in terms of the source quality. I'm no tech expert and I don't know why the 1s and 0s don't all sound the same but (much to my horror) I have discovered I can hear a significant difference between optical and coaxial ouputs (coaxial from the ZP90 sounds better in my view - clearer and also fuller and less 'brittle'; low notes on violins or cello are a great way to test this!). However....
- This does depend on the DAC. Some DACs are much better at handling the 'jitter' (or timing errors) than others. The better the DAC, the less sensitive it is to the source quality in my experience. If you only use an entry level DAC, it is quite possible it would sound better accepting a digital signal from a decent quality CD transport than the feed from your Sonos (and I'm taking it as given that the files being streamed are lossless - not low bit rate MP3s!)
- In my view, sound quality can be improved by inserting a reclocker in between the Sonos and the DAC. This takes the digital feed from the Sonos, applies its own clock to the signal (which is supposed to be more accurate and has fewer tasks to perform than the internal Sonos clock) and then supplies the re-clocked (i.e. lower jitter) bit perfect signal to the DAC. I don't own one of these devices but I have heard them in others systems. The effect is subtle but quite audible and well beyond the 'snake oil'/placebo effect I've found with some other 'upgrades'!
- Even more annoying and (to me) surprisingly, I have been forced to accept (following experimentation) that the choice of digital interconnects/cable also impacts the sound quality. I've tried a small sample of what's on the market and have settled quite definitely on the Black Cat Veloce digital coax cable. I found this to be significantly better than others I've tried from Chord and Wireworld in a comparable price range (somehow just sounds fuller, less 'edgy' and more musical). If you do use a reclocker, only the cable downstream of the reclocker (i.e. between the reclocker and the DAC) makes a significant difference to the sound - you need a really bad cable to ruin the data integrity of a bit perfect signal but a good cable can, in my view, reduce jitter. (Note that I believe the choice of DAC will have a much greater impact on the sound than the choice of cable - cable is the last 5-10% of difference only)
- Audiocom (UK) and Cullen (USA) both offer mods to the Sonos that improve the quality of the output (albeit they probably void your Sonos warranty). Audiocom takes the route of upgrading the power supply and reducing associated noise. Cullen takes the approach of upgrading the Sonos internal clock. I have not heard Cullen's mods but I do have the Audiocom modded ZP90. To be honest, I was quite disappointed with the result. In my opinion clarity is improved vs the stock ZP90 but only very subtly and not v good 'bang for buck' vs the potential improvements from spending this kind of money elsewhere in your system.
- My modded Sonos still doesn't sound as good as the USB feed from my computer after it has been fed through a JKenny MK2 Modified HiFace USB to SPDIF (i.e. digital coax) converter/reclocker. Low level details are slightly smeared/harder to pick out on the Sonos and the sound is not as rich/musical. It would be interesting to see if using an external reclocker for the Sonos evens the score, which I guess it theoretically should do!
All in all, the digital out of the Sonos fed into a good DAC is very solid 'mid fi' sound quality. To approach true 'hi fi' though, you either need to spend more on the soure (E.g. Linn, Cyrus or Olive streamers as mentioned already) or add mods/upgrades to the Sonos signal. However, as with many audio upgrades of this nature, I believe the law of diminishing returns sets in very quickly so you ultimately need to decide how far you really need/want to go and how much you are prepared to spend. This decision also needs to be made in the context of how a similar spend could benefit other parts of your system, potentially with greater effect (e.g. dac, amp, speakers/headphone upgrades).
Just my two pence worth...!