I believe it to be a monitor based unit first, perhaps pa but has very little hardware which comes with solo instrument amps especially acoustic. So my belief is no it is really meant for use as a monitor at first conception and the buskers whos needs for solo battery amps is the direct outcome of buskers needing light, long battery life, for mobile use which has expanded into more solo performances. I think that solo performance was a secondary plus for Bose. but not its original focus.
So, it's really just a powered speaker with long battery life? Loud enough to be a pa main speaker? Pretty expensive for a monitor speaker.
If you check out these pages
You'll find that many of the use-case examples show solo performers using the S1 Pro as their personal portable PA. Yes, you can use it as a stage monitor as well.
Looking back over your previous posts, it looks like you have an L1 Compact. As a result, you may not need an S1 Pro unless you want a smaller, lighter alternative that can run on battery power.
Here's a comparison chart.
For intimate concerts where even the L1 Compact seems too big, the S1 Pro fits in nicely. It's unobtrusive, and not having to hunt around for an AC power outlet in someone's living room is a nice bonus.
Do you need a personal PA for solo performer type gigs?
This S1 pro lacks the essential bells and whistles for solo amps eg. feedback eq, phantom power, delay, reverb, chorus, DI, notch filter, looping, ... ... these items usually come on solo amps for acoustic electric. I use the S1 for solo busking, but use pedals to compensate for the lack of these normal features on solo amps. not present on the S1. Using a PA or the Ts / mixers is another solution. Bose I believe sells the S1 with the hope that the S1 is used in combination with the rest of Bose products, therefore the S1 is lacking the features found on solo amps for acoustic /electric. I don't believe Bose fully intended the S1 as a totally independent solo amp.