Keep it simple: Connect the player straight to the speaker - nothing in between (other than cable and Sonos line-in adaptor). Try adjusting the line-in delay on the Era -start with maximum. And, if you’ve not already tried them all, try each of the line-in level settings to see if one does result in no distortion. Ensure all plugs are fully inserted into sockets. 

This is what I tried originally- just plugged player straight into Era100 and adjusted the line in levels. Tried them all and still distorted. Changing the delay settings has made no impact either. 

Assuming the 2xRCA pin to 3.5mm stereo jack cable is fully inserted at both ends, and it is a genuine Sonos line-in adaptor, I think a call to Sonos support is needed. 

Thank you- I had not considered the Sonos was the issue. I have just tested the line-in on my Move 2 (which I had forgotten had this handy feature!) and there was no issue so it would appear the Era 100 is the problem. 

I contacted the helpline and they advised doing a software update which has now resolved the issue.