Why would I ever want the 75? Everyone in my house speakes English.
I want to put a second speaker in the lower level of the house. AO has the 73 for about $9. Do I need an expansion module or can 2 speakers be directly connected to the M1?
Yes, you can put two speakers in parallel, which will give you a 4 ohm total impedance (the lowest you can go). You could put more then two speakers but you would then need to use a series/parallel arrangement in order to stay above 4 ohms.
Speaker size makes a difference in sound pressure generated. The larger the speaker the more sound pressure generated. A 40 watt paging speaker will be seveal time louder than a small 15 watt speaker.
Make sure the speakers you use are rated at least 15 watts or you run the risk of burning them out from the amplifier output.
If you must use a less than 15 watt speaker, add a current limiting resistor (ex: 5 ohm, 5 Watt) in series with the speaker lead. This will also cut the volume down slightly, but protect the speaker from burn out.
A 40 watt speaker may be capable of being louder, given that it can handle more power without burning out. The wattage specification for a speaker only says how much power it can handle without burning out.
What determines loudness of a speaker, for a given amount of power, is the sensitivity rating of the speaker. The wattage rating of a speaker has nothing to do with the sensivity rating; they are not related in any way. For a given power level, 15 watt speaker may sound louder than a 40 watt speaker; again, it depends on the sensivity.
Now I do agree with your statement about limiting the power to a speaker rated at 15 Watts or less for a 12V, 1 amp speaker circuit. This circuit would have the ability to drive 12 watts (12 Volts X 1 Amp). If the speaker can't handle the wattage, it will fry.