I was trying to simplify the answer based solely on the prinicpal of the load presented to the amplifier vs. the output. (A simplified version of ohm's law again - the same power output at a lower resistance means an increase in amperage, so yes you've got it - the less resistance the more current needed from the amp) In reality though you can calculate the load of a speaker (it's rating at 8, 6, or 4 Ohms) as something called a "reactive" load. This means unlike a resistor which has a static value, the loads charateristics change depending on the frequency of the signal. So an 8 Ohm speaker is roughly 8 Ohms over a given frequency - in reality it can vary greatly.

Again, this is a simplified version, but a 6 Ohm speaker should not present a problem to an amp set to 8 Ohms, though it will probably be less efficient than your "rated" output.

I'm sorry if this is confusing - I checked the board again after a few glasses of scotch, so my answer may not be as clear as I intended.

The general rule of thumb seems to be that receivers with 4/8 ohm switches will work fine with 6-ohm or 8-ohm speakers when the switch is in the 8-ohm position. The switch helps manage the current flow increases caused by low-impedance (4-ohm) loads. This is typically done by lowering the voltage supplied to the amp, essentially "throttling" the amp back.

I finally started understanding resistance after hearing a water flow analogy. If you have water running through a 1inch hose a certain volume will create a certain pressure. If you cover half of the hose end with your thumb, you will restrict the flow ( increase the resistance)so the same volume moving through the hose will come out the end in a smaller stream but with greater force. If you connect a 1 1/2 inch hose to the one inch hose, the same volume will come through the 1 1/2 inch hose, but it will come out with less force. to achieve the same force through the 1 1/2 inch hose you would have to increase the volume of water because the resistance is lower.
It's basically the same with the load on the amp, the lower the resistance, the greater the current required to achieve the same force.
I think I understood this better before I tried to explain it, but i hope that helps some.

KOYAAN,

Thanks for that excellent explanation. It definitely helped me to understand the resistance better.

An even easier way to think of why too little resistance is bad: The closer you get to 0 ohms, the closer you are to a short circuit.

Many amps see 2 or 3 ohm loads as a short circuit, and they are unable to provide the level of current, thus they max out the amount of current they can send which:

a) builds up excessive heat
b) causes distortion to occur (which is worse for your speakers than for the amp)

Many amps have either a fuse (typically in older models), or protection circuitry that disconnects the output in case this happens. The older models obviously are a pain these days as you have to open them up to replace the fuses, but the newer ones usually just need to be powered off and back on to reset the output. Some professional amps will come back online when they detect a more appropriate load.

I actually have access to a number of amplifiers that will happily take a 2 ohm load, just none of them are in my home. Gotta love Crown amps.

I got in a little late on this...but just the same...

Speaker loads are not purely resistive, they are reactive. They consist of the inductance and capacitance of the speaker coils and crossover components (and the wire itself!) and the tiny amount of resistance in the wire and any crossover resistors. Inductors and capacitors are frequency sensitive, and therefore the load presented to the amplifier varies greatly with the frequency of the audio signal. A four ohm speaker might present as high as a seven ohm load at one frequency and as low as 2 ohms at another. Because most musical content is a complex combination of frequencies the load the speaker presents to the amplifier is also complex.
Designing speakers and amps would be a lot easier if the loads were purely resistive. Everything involved would be fairly predictable.

Why do you want to use two speakers for the center? You're unlikely to get a significant increase in acoustic output unless the amplifier is fairly powerful and can perform better with the change in load impedance of the new setup. A possible unwanted side effect will be created by the radiation patterns of the speakers interfering with each other. This could actually muddy up the sound. And if you put the centers side by side you will have increased the width of the soundstage. Maybe good if you have a really big screen. Otherwise probably not.
If you need more volume from the center you would best get it with two speakers driven by two amps. Or by a more efficient center with your present amp. Or a more powerful amp driving the center, assuming the speaker could handle the additional power. And then the outputs of all the channels must be recalibrated to the new center configuration. After which something else may not seem loud enough.
It's a tangled web, indeed...