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