Hello @EX242, welcome to the Sonos Community!
I am sorry to hear you are having issues getting your Sony PS-LX350H turntable with a pre-amp to play on your Sonos System with your Sonos Port.
I just purchased a Port to pair with a Turntable I have (SONY PS-LX350H) with a pre-amp. I can barely hear sound out of my Sonos System with volume maxed out 100%.
From what you shared, it sounds like your pre-amp is not working properly. Make sure it’s turned on, remove and replug the power cable as well.
Do I need the AMP? It looks like it just for non-Sonos speakers.
You do not need a Sonos Amp, since you have a pre-amp on your setup. Your turntable should output to the pre-amp and the pre-amp should output to Sonos Port Line-in.
Since you have a Sonos Five you can try connecting your turntable to the Line-in port directly to test that out. You also have the ability to connect it directly to your Sonos Era 100’s if you purchase our Sonos Line-In Adapter.
You can try adjusting the Sonos Line-in settings via the Sonos App and more importantly the Source Level setting.
I hope this helps.
Post details on the pre amp you are using.
You need an external phono preamplifier for that turntable.