The Amp can be used to power third party surround speakers for the Arc, but when the Amp is used in this way, the sub output and line input are disabled on the Amp. In this setup, your best option is to use the Sonos Sub.

The only way to use a third party sub is via an amp that is not powering anything? Id rather run a quality sub over surrounds anyways.

You simply can’t use a third party subwoofer with the Arc, with or without an Amp.