There are two steps to this i’d suggest.

First set up a Stereo pair for the Two Era 300s in the Sonos App.  This is a simple process that takes a couple of minutes.  Open either one of the ERA 300 in settings and there is an option to create a Stereo pair, it will identify a matching speaker and walk you through the process.

Secondly you will nee a Sonos USB to Line in dongle, this must be bought from Sonos as it is designed for this specific use. 

Phono Adapter

The Line out from the Pre amp will need to be wired into this dongle.  You may need an RCA to 3.5mm jack adapter, or lead as the Sonos dongle only accepts a 3.5mm input(same as standard wired headphones).

Wire this up to any Sonos spekaer that has a Line in or USB input.  In settings be sure to high in setting for the Line in input and play some vinyl.  If you have other speakers you can share to them too or even play the turntable sounds to another speaker and play a different source on your ERA 300s.  Total flexibility.

There is also a good FAQ and instructions on Sonos help

Listening to Vinyl on your Sonos System

Thanks, that's as far as I've gotten except for purchasing an RCA to 3.5 to go from the turntable to the dongle. 

I've already set them up as stereo for playing over Bluetooth, so no issue there. 

My only question with this set up is if the sound quality will differ from left to right speaker if only one is 'wired' and the Sonos system is 'casting' to the paired speaker.

I'm after as high quality audio as this system can put out from the turntable. 

Any insights?

Not sure why you think the sound quality would be different from left to right?

Sonos is capable of carrying and reproducing far higher quality sound than you can get from a turntable.

Using the analog input is likely going to produce higher quality sound into the Sonos system than your Bluetooth link so you are headed the right direction.

Thanks Stanley, I'm new to this. My assumption was that not having both speakers hard wired in would impact the audio quality. Good to know that the Sonos in stereo mode, with one wired in is going to produce great sound through both. 

The concept of hard wiring is sort of irrelevant with Sonos.  You can connect every speaker via Ethernet cables but the Wifi connection to the speakers holds the same info, so unless you have areas in your house without wifi coverage it can help to wire speakers but with the latest speakers and their wifi 6 cards it is becoming less of an issue.  The quality of sound transmitted over wifi and ethernet is the same and is well above that of Bluetooth and is capable of HD audio transmission.

The connection from your turntable to the dongle-speaker will be digitised into the Sonos system and shared to all speakers equally so it doesn’t matter which speaker it runs to either.

It’s this high quality wireless system that is synchronised with all the speakers that sets Sonos apart and why they protect these patents so eagerly.  It is also why when you are playing Vinyl you will find ther is a delay in the sound you hear from the record and needle contact to that coming from the speakers as Sonos has a buffer to allow for the digital conversion and then the sync to all the system.