What will you use the Amp for if you purchase the Arc?

The Arc is the ‘front’ three channels of a Home Theater setup, it is normally connected via ARC or eARC. It can not be used as a center channel only device.

If the Amp will be used to drive the surround signal to the connected speakers, it is ‘bonded’ to the Arc wirelessly, via a 5Ghz channel created by the Arc. 

the intention was as a center channel- I currently use an old LG soundbar connected via optical - the intention is for clarity on speech and enhanced  sound.  Front speakers are Definitive towers connected to the amp, surround is via sonos ones

Sonos doesn’t provide any solution for a disparate center channel speaker. All Sonos soundbarrs are front right, center and left channels. For the Sonos Amp, such as your current install, you have front right and left, with an interpolated center channel.