I never did answer this one and it's probably hard.
Amps can have A/B speakers where you can select which ones or both to use.
What you really need to know if your going to use 2 sets of speakers is: two 8 ohm speakers in parallel equals a 4 ohm speaker and two 4 ohm speaker in series equals an 8 ohm speaker.
A speaker will typically be rated in SPL for 1 Watt or Sound Pressure Level. The higher the number the more efficient the speaker.
Amps will be rated as 100 W/channel, both channels driven into 8 ohms. Any other ratings like peak Watts is just GARBAGE.
A 4 ohm speaker requires more current output from a amp whereas an ohm speaker requires more voltage. So if your going to parallel or series connect speakers the amp should be rated for the anticipated load.
I'm not sure exactly what you expect out of a ceiling speaker. I'm asuming nice music and not to loud.
There are devices called "L-Pads" which are effectively volume controls at the speaker level. e.g.
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And there are devices that will allow selection of many speakers and they will keep the impeadance constant.
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As I said, the Bose Ceiling speakers are pretty nice. My local Panera Bread has them in the ceiling.
Ceiling speakers won't give you the benefits of having an enclosure.
100 W can be pretty loud. I'd look for something in the 60-100 W range.