Current, Amperage

If 4, you need to check the 4 ohm rating. Some double, other no where near double power to 4 ohms from 8

Just because an amplifier can't double its power when impedance is halved does not indicate that it can't behave as a voltage source.

@elizabeth 

Thank you. 
I agree, and this IS the point of the discussion; What is enough [we know too little is bad, but how much overkill does one need ]?

I had a McInotsh MC252 - 250 WPC on either 2, 4 or 8 ohms, and I loved it, but do I need that again in a 12x12 room? used for $3500 for my type of setup is a lot. I don;t need the 'Wow' blue lights factor again :)

Can a Rotel 200 wpc or something else provide enough [that will be good] without having to spend more than $500-1000 used? 

Thanks; this is an interesting topic for all of us that have decent systems but maybe not more than $5-$10K into them. 

It's more about synergy, not math.  I have a 300 wpc Mac but usually am using my 10 watt triode.  Running out of power can ruin a speaker and/or your experience but quality is much more important than quantity.  

My math was a generalization but you do need a frame of reference on these things.  Generally speaking you gain 3db for doubling your wattage is a great way to understand why going from 40 to 60 watts is virtually inaudible all other things remaining equal.