Found a manual for the cd player, the wharfdales are connected to the no2 speaker terminals, if that helps ‍ 

Perhaps just use an RCA switch with 2 inputs and one output. Connect the Denon RCD-M38DAB RCA output to the switch input ‘A’ and connect the Turntable/Pre-Amp RCA output to switch input ‘B’.

Then cable the RCA switch output to your Sonos Era 100 USB-C port, using an RCA to 3.5mm cable into the Sonos Audio adapter. The adapter is available here: 

https://www.sonos.com/en-us/shop/sonos-line-in-adapter

Thanks I thought that had solved it for a minute, but I'm not sure.

The issue as I understand it (I may be wrong ‍ ) is that there are 4 speaker connections on the the back of the Denon, 2 (+ & -) for left & 2 (+ & -) for the right. I think, that in order for all sources connected to the Denon to be played through the speaker, it would have to be either connected to all 4 outputs or the headphone jack on the front. To make it even more complicated (for me) the speaker outputs on the Denon aren't for plugs, they're the type where you tighten them up to trap the exposed speaker wire

I’m not sure what you’re referring to here the Denon RCD-M38DAB connection to the switch will be from the RCA line out. See the highlighted photo below… the speaker outputs will have no bearing on the playback to your Era speaker via that suggested RCA switch and your Turntable/Pre-Amp will also be directly connected to the same switch too as its second input, allowing you to play both on your Era 100… you just need the cable and Sonos audio adapter as mentioned to go from your chosen switch to the USB-C port on the Era device.

Thanks I thought that had solved it for a minute, but I'm not sure.

The issue as I understand it (I may be wrong ‍ ) is that there are 4 speaker connections on the the back of the Denon, 2 (+ & -) for left & 2 (+ & -) for the right. I think, that in order for all sources connected to the Denon to be played through the speaker, it would have to be either connected to all 4 outputs or the headphone jack on the front. To make it even more complicated (for me) the speaker outputs on the Denon aren't for plugs, they're the type where you tighten them up to trap the exposed speaker wire

I’m not sure what you’re referring to here the Denon RCD-M38DAB connection to the switch will be from the RCA line out. See the highlighted photo below… the speaker outputs will have no bearing on the playback to your Era speaker via that suggested RCA switch and your Turntable/Pre-Amp will also be directly connected to the same switch too as its second input, allowing you to play both on your Era 100… you just need the cable and Sonos audio adapter as mentioned to go from your chosen switch to the USB-C port on the Era device.

 

Ah, I thought that the Aux 2 output connection highlighted would only output sound from whatever is going to the Aux 2 input the turntable can be connected to the Aux 2 input, but I assumed that the cd player itself only output to the speaker outs, but I can test that to be sure, thanks again

Just tried playing a cd & connecting headphones to the aux 2 output & it works, so I was indeed wrong, looks like I can connect the turntable preamp the aux 1 input as before & the Era 100 to the aux 2 output with the line in adapter & all should be good

@MrDeps

Yes, of course you can do so… the aux2 is what we called a classic tape deck connector in the past with in- and also output to record from each source.

So as you already realized you won‘t need a RCA switch because your Denon itself is the switch.

Probably you also should be able to connect another source device to aux2 input that would be passed through to aux2 output if you select aux2 as source with the denon. 
In the end you would be able to get internal CD, phono and another anlogue device signal to your Era100. 

@MrDeps 
Yes indeed, no switch is required if you run the TT/Pre-Amp via the Receiver/CD player. I was (mistakenly) assuming you perhaps might not want to power on the Receiver/CD player each time when using the TT (or vice versa) and keep the two inputs to the Era 100 entirely separate, but it’s absolutely fine the way you’re suggesting and cuts out the switch and some cables, so I’d go with that.

Thanks for all the help folks, as soon as I've moved on my wharfdales I'll get an Era 100 ordered