Actually, the speakers work perfectly for the intended use, which is a "home environment".

To expect such small speakers, and relatively low power (easily read in the specs) to work on stage is going way beyond the keyboard's intended usage. I'm sure the person who sold it to you, if they were using it on stage, would have (or should have) mentioned it.

I've had zero complaints of PSR-S910 speaker quality and performance from home users.

You'd have been better off keeping the PA-800 and getting it refurbished.

Ian