no
the only way to get 5.1 with Sonos is to use one of their sound bars
You can use an AMP, but you'll need to bring your own front speakers, and then run speaker wire back to the AMP. One of the big downsides is this is only 4.1, since you can't have a front center speaker, the center channel will be split to front L&R. Without the dedicated center channel, it can make dialog harder to hear.
The sound stage on the Arc is actually quite wide, with the side firing speakers. Using a channel check audio file that runs each channel individually, the front left and right sound as if they're a couple feet on either side of my TV, even though the Arc is not as wide as the TV. The beam is also not bad.