Sonos doesn’t have an amplifier that can be dedicated to only a subwoofer. Have you checked with the folks at Sonance to see if they have any information for you?

And, on rereading your question, as long as the subwoofer is self powered, as I assume the Sonance is, you could put pretty much anything there. All the Sonos Amp does it put out a line level signal for the Sub, there is no amplification on that output.