If, as suggested, optimum fidelity is the aim then running the files through a DA converter attached to the iMac then back through an AD converter is the last thing one should contemplate. 

Abandon the idea of using Line-In.

Simply do a one-time down-conversion of the files to 24/48 ALAC on the iMac, then index them into the Sonos library in the conventional way. The benefits of any careful remastering of the files will be preserved.

This is the only sane and sensible way to play the files on your Mac. It is better in every respect than your current method, or any variation on it using an external DAC and line in. 

I am gonna add fire, to the question if using a Zen DAC you will be able to play 96 Hz through the SONOS AMP’s analog input, the answer is straight no. S2 the operative system of SONOS tops at 24 bits/48 Hz, this is the maximum rate that the analog input may (or may not) convert the analog signal from any external DAC.

To the not clear question if an external DAC will play trough the analog in, the answer is of course and I am gonna add you may like the results. 

As an AMP user I have used the analog in with several devices none more expensive than the AMP itself but neither of bad quality, near every time the presentation through the analog in seems to win, this is weird considering I have ripped CDs on a wireless hard rive and I feel more pleasant the CD player presentation. This gets more obvious using a proper analog source as a turntable, many times the subjective stereo with the turntable was actually better or at least easier to pinpoint with the turntable than lossless streaming, later I tried a cheap Cyrus DAC headphone amp from my computer and bingo it created a quite nice reproduction with a stereo image that really pleased me. When I am doing chores I ask Alexa what I want to hear through the house but if I am seating using the amp I actually use the analog in with a dam cheap DAC. My point is, I do know that one DAC converts to analog an another one after the input creates a digital signal but in short the results should be equal or worse and they are not, they are different and you may like that difference. I do.

I can start speculating why or how,  but it would be fruitless. SONOS is quite secretive and none of us knows exactly how is the SONOS AMP designed, and the difference between the streaming section and the analog in,  hence I cannot prove or disprove my opinion.

External DACs are a thing for a reason as they perform differently and this performance is objectively measurable in thinks like jitter, but without more information from SONOS what I can add is that a DAC as source can produce a different playing experience than SONOS playing the file itself. 

“external DACs are a thing for a reason”.  If there is a reason, it is that one DAC may perform better than another one if it replaces it in the process of getting from digital data packets to music you can hear.  Some may dispute this, and my personal opinion is that in many cases the difference ‘heard’ is caused by expectation bias.  But let’s go with it for a moment.

I have a set up that includes a Connect and HiFi.  I use digital out from the Connect into a Cyrus DAC/pre-amp.  So the Connect’s DAC is replaced by the Cyrus one.  This, to me has at least the potential to improve the sound.

But this thread is talking about sample rates etc on a conversion that enters the Sonos system as an analog signal.  This still seems nonsensical to me.

No it isn't,  this is about connecting a DAC as a source. 

I don’t know what that sentence means.  Please excuse my ignorance and explain.

Can I add a dac such as the iFi Zen DAC to enable me to get higher than 96khz ? was the initial question. Although the question has a mistake as SONOS tops at 48/hz The meaning of my answer is  that he can use any external DAC That can read such files or other not supported by SONOS and then send it through the line in. I am adding my personal opinion that oddly it can work in a way that “I” like more than SONOS playing the files or streaming itself.

A chain that includes a 96/192kHz DAC, a 44.1kHz ADC and a 44.1kHz DAC doesn’t strike me as the most unsullied way to go, but each to their own.

Some people swear by vinyl and tube amps, which each adulterate the sound in their own special ways.

I don’t think that makes the DAC the source.  There was no disputing that an analog output from any DAC can be used in the line-in.  Because it’s analog.  What was questioned was the the OP’s apparent belief that the sample rate etc on the original source was going to make a difference.

I am perfectly happy to accept that you subjectively prefer the sound produced to that produced by alternatives.  I suspect we actually agree on much more than we disagree on.

Yes, what I mean is that is not the sampling rate, is the DAC itself and its quality or the way it handles files that SONOS cannot handle that would make a difference, there is a lot to know about the AMP design so I feel pity that everybody has turned this question into a HI-Re vs Redbook when it is not.

In fact we should relate this  to a different eternal Hi-Fi debate, if separates are better, although I am not unaware that SONOS is a digital amp and converts everything to DIGITAL, my hypothesis is that I found that the streaming section of the amp can be bettered off by the line in despite adding an extra analog to digital in the way. 

Playing local files, or streaming from an online source, requires a decode. After decode, the audio would be in PCM, the same as the output of a Line-In ADC. If the latter sounds ‘better’ then it calls into question the quality of the original content encoding (or, in the case of local files, of any CD ripping).

FWIW adding an extra analog to digital (or DA) conversion can only degrade the sound. It’s impossible to improve it, unless of course there a subjective preference for any ‘colour’ (distortion, noise) introduced.

Wonderfull! How do I improve it? I love the placebo police. Don’t you think I did not think about it? So what do I change? The ripping software? The hard drive? The Router? Have you seen the price of a so call audiophile wireless unit? Following that path will end up in an audiophile nonsense race that you criticise so much, specially because I do not know if the so call colouration may come from the AMP itself while decoding or better said transferring the music files. I am actually in agreement of your statement, it shouldn’t but it does at my home. It is actually way cheaper wiring the SONOS straight with a cheap DAC an enjoy it that trying to answer the questions you are making, I am not Amirm from audio science review I just answer as an amp owner the primary question of this thread. I am sorry if you do not like my answer because it doesn't fit to your logic, it doesn't fit mine either but I just drop it. I reiterate my first stamens that you will not get Hi-Re but the reading of whatever the guy wants to fetch from his laptop may actually play better “for him” in that way. 

No ‘audiophool’ snake oil required. A secure ripper will make a better -- possibly even perfect -- transfer of the music data off the CD. This has been well known for a long time. Lossless compression retains all the original data. Also a well established fact.

But do whatever appeals to your senses. If you prefer a euphonic colouration introduced by your DAC then by all means insert it into the chain. 

I do not need you wise help to tell me that. If u wanna patronise you better research what you are talking about, it is said that the SONOS amp uses a Qualcomm chipset, this chipset doesn't need a DAC I actually researched that but its got its own processing because it has to implement the SONOS software to convert the streaming into analog out for the speakers. The details of this part are unknown so I won't speculate any further unlike you who speak like you designed it. You have to prove me in testing how good this process is before saying with so much arrogant security that this processing in the digital domain is indeed superior or less coloured or produce less jitter and so it goes to the one made in hardware by the separated DAC that SONOS should have implemented in the Analog in. So much people talking like audiophiles about the purity of sound path when nobody has opened the device to work out what that path is. Finally the AMP is not a speaker is an amp with Streaming, HDMI in and Analog in your security that there is no difference in the inputs quality or affirming that the streaming part is “the best one and shorter play path” is as arrogant as turntable owners arguing about the best MC cartridge. I will not use the word coloration to describe my experience because I do not pretend to know what goes inside the amp unlike you.

This discussion was about the merits of adding an extra digital-analog-digital stage on the front of whatever goes on inside a player. As I said, if you subjectively feel the effect is more ‘musical’ then go right ahead. 

Perfect 

For the matter changing the ripping software, as I said some comments above, was a rhetoric comment, I am a SONOS owner I do know what exact copy and iTunes are. I like the word musical better than coloration though so peace flower from here. About that, the AMP uses 3rd party speaker so every single owner of an AMP by definition got a different “Coloration”. Coloration that unlike the SONOS 5 for example we cannot correct as True Play doesn't work with 3rd party speakers, for the matter what I am saying is that the Line in processed in whatsoever sampling rate they use is surprisingly able to show differences between digital sources let alone a turntable. If somebody has a situation that requires a file SONOS cannot read I am not gonna tell him not to use it because my owner experience with the Line in an eternal DAC  is good really good. I had wondered in perfect agreement with you that my DAC may have a frequency response that works better in my room (coloration as you say) among a lot of theories  but researching more on the matter is too audiophile for me to pursue…

Be in peace! I have nothing add on the matter.