Can I swap out my unpowered stereo amp speakers with powered ones using a Bose Companion 2 Series III (i.e. will there be issues having both the stereo amp supplying power and the Series III supplying power at the same time)?
The only way I can think of connecting them, is if your amp has a second zone or headphone out. You would likely be going from the headphone out into the Aux in. That way it's not powering the Companion speakers, just providing the source.
If you try outputting like other passive speakers, not it won't work. You wouldn't have the right inputs, plus I think there would be a power issue if you could.