Can you hook it to a Tape-Monitor loop, that is usually the best option.

A pre-amp Out would work but you’d end up with two volume controls then,

this one doesn't have tape monitor out

has a Media player out and zone 2 and 3 out?

Any active output would work if you can reprogram the AVR to send your sound to it. My Denon’s manual was less than helpful in that but it is a starting point.

If you have a CD player hooking that to your Sonos and making sure you can play from it would assure you you had the Sonos settings correct. Then go back and fight the AVR configuration.

OK, some slight success, going from media player out to Sonos connect in (and connecting an optical cable) has that working with the 2 x play 5 's in Stereo. 

Swap to phono, and sound is awful, muddy, quite and not in Stereo. Do I need to remove the pre amp (I'm sure my 80s TT won't have one) or is it something with the line levels?

With a phono you need One and ONLY one phono preamp.

The phono preamp as suggested provides amplification, at varying levels for MC and MM cartridges but also provides the RIAA equalization curve. No curve applied to the signal and it sounds bad, curve applied more than once sounds bad.

So pick just one phono preamp, be it in the turntable, an external box or the one usually built into the AVR.

PROVED Solution to connecting Denon AVR  to Sonos Inputs:.

(I have an X1800H which has pre-outs.Other builds of this model do not have them. Try Crutchfield or simply just be sure your AVR has a pre-out) 

Like many of you I wanted to share the main zone/ room’s audio to my sonos system so I could use the turntable to hear in the gaage, patio, dining room etc. OR stream from the computer desktop (via hdmi to the AVR) a concert. Denon wants you to use Heos speakers a system so similar to sonos that Sonos sued and won.

After a month of research and experimentation including calling Sonos and Denon Tech support and studying the uselss manual-- I called my AV guru. I  told him my Denon pre-outs were weak garbage ( I was mad at this point). For us older audiophiles, gone are yesteryears  “line-outs” on every receiver that were a constant usable level for almost anything including tapedecks. Today’s pre-outs are apparently made for a secondary amplifiers in another room. In Denon’s case that that is bizarro becasue adjusting the main zone volume changes the volume of the pre-outs so affects the other room(s). Plus who wants to route WIRES to another room especially when we have SONOS?

I was trying to find a line out amplifier that would not blow out my sonos Connect’s inputs. I found one. It worked but was too weak. It is advertised at 20db gain. I found one. Thre arent many. Dont bother. The best of these is a 20db gain. My AV Guru said “Try a mixer”. This is a huge area. Beware! Based on my exerience you want at least a 30db gain not twenty. I found one. Drum -Roll please:  The Rolls MX44.  To review: All your devices such as TV, computer, turntable, tape deck, DVD/BluRay, whatever all play into your AVR and out the ever so weak pre-outs. My pre-outs connect to my Sonos CONNECT inputs. The gain on my mixer is all the way up at 30db gain making it usable by the Sonos. To maximize this gain be sure your Sonos Input level is set at #1 using the mobile app -just below for AV component.

The Rolls MX44 is currently out of stock (I bough the last one a week ago!) but is available at same $99 price all over the internet.

https://www.amazon.com/dp/B09ZJBQSWX?psc=1&ref=ppx_yo2ov_dt_b_product_details

DO NOT BOTHER WITH THIS. ITS 20db  OUTPUT IS NOT SUFFICIENT:

https://www.amazon.com/dp/B08L4WYSZ4?psc=1&ref=ppx_yo2ov_dt_b_product_details

For the Denon AVR X4000, I suggest sending Zone2 or Zone3 output to the SONOS Line-In. These outputs have their own Volume control that you would set and forget.

Note I’m not familiar with the details of this receiver. Some receivers will not convert digital audio sources to analog for these outputs. This is easy enough to test. First, send the tuner out, then try a digital source, such as an HDMI input.

The problem with all Denon pre-outs is they are not strong enough for SONOS CONNECT to amplify to a decent level. You need a MIXER with GAIN in between. . Similar in principle to a Preamp for phono. Digital and analog is not a problem as AVR convert all digital sources to analog in order to put the sound into the speaker terminals. 

Have you tried the Zone3 or Zone3 output? I’d be surprised that between the CONNECT’s Line-In adjustment and the Zone output Volume control you couldn’t get a satisfactory level.

Thanks for the input one and all. I'll look into this when back home in a few weeks.

Denon pre outs are too weak unless you have zone 2. Set zone two to fixed (70).

Using OPTIONS button on remote select all zones. Select Start. This combines zine 1 and two. Important as it allows zone 2 to output both analog  digital sources which only feeble main zone will do you won’t find that fact in manual or any thread here nor will you find you can eliminate audio delay echo by muting. Zone 2 continues unaffected

set Sonos line input to at least 6/7. 
 Why? Denon owns Helios and wants you to buy their system not Sonos….

For more details see my other post:

Proved Solution outputting Denon AVR to Sonos inputs.