poor sound quality playing vinyl with Port

Sonos line level input  needs to be set around 10, check this first.

Thanks. I’m not familiar with Sonos line level. My pre amp doesn’t have a selection for line or phono.

It may have a MM/MC switch or jumper which does need to be set depending on your cartridge type.

Well… 

Is the cartridge MC or MM ?

Is the phono stage actually working correctly? 

You also need to check the tip of stylus to make sure it not worn and no damage to the cantilever. With the cartridge disconnected you need to check continuity of the tone arm leads. Then tracking weight, arm height and cartridge alignment, anti skate and then revisit some of those again.

Finally if all OK just remember a £500 turntable will still sound worse than a £50 CD player. A vinyl cleaner goes a long way to help older records and even new ones.

EDIT you can test the Ports line inputs using a CD player or something that has a line out such as a tuner,  cassette player or even an iPad etc.

Your existing amplifier… may have either a pre out or tape out, you could try connecting these to the Port’s in and see how it sounds. Keep the volume down low initially in the Sonos app in case your amp sends a full signal.