The Amp doesn’t have a separate connection for a center channel speaker. It only has left and right channel speaker connections and a subwoofer output. In a home theater setup with the Amp, you can only achieve a 2.1 setup with a phantom center channel. With a second Amp you can add two surround speakers for a 4.1 setup.

So currently, my amp has the hdmi arc connected to 

y smart tv. A Bronze 50 connected to the left and right channel, and my Marantz turntable with inbuilt preamp also connected to the amp. 
ti enhance this setup what are my best options? A Beam/subwoofer?

Your best options would be either adding a Sonos Sub and/or adding a second Amp and two passive rear surround speakers like I mentioned above. Since you have a turntable, adding a Sonos sound bar like an Arc or Beam is not a good option unless you are willing to purchase another Sonos device with Line In like a Port or Five. When you use the Amp as surrounds, it disables Line In.

Thank you so much.  A-lot to think about. Sonos has been great. It’s been a great gateway back into the hobby. 

I think the sub sounds like a good idea. I use the current up to obvs listen to by vinyl, streaming Spotify and tv.  It a big living room. Only 16ft by 11ft. 

Given I have the Sonos Amp. For best results will I need to physically connect the Sub woofer into the back of the Amp. Or will it be ok to place wirelessly?

The Sonos Amp will auto-communicate ‘direct’ with the ‘bonded’ Sub. It’s usually easier/best to let it do that using the built-in 5Ghz ad-hoc wireless adapter connection, rather than wiring things via the LAN.