The lower the impedance of the speakers, the more current the receiver has to supply. V=IR and V (voltage) is constant for a receiver, so when R goes down (note that impedance is not quite the same as pure resistance (R), but close enough for a government job), the I (current) must go up to maintain the relationship. The extra current requirement is what stresses a receiver if it is not specifically designed to handle that requirement.
When you use a switch to change from 8 to 6 or 4 Ohms, all it does it reduce the rail voltage (the V), which in turn reduces the current requirement. That limits the max volume but is necessary to meet UL requirements for fire safety. The usual recommendation is to just leave it at the 8 ohm setting but avoid cranking it near the max where the current requirement would approach the limits of the receiver. If you find you need to crank it near the max to achieve the volume you desire, then you need a more powerful amp.
And No, changing the impedance selector will not give you more SPL. It will give you more power but that is simply because more current is required with the lower impedance. Two speakers with the same sensitivity will play at the same SPL but the lower impedance speaker will require more current (and hence power, because it is all related) to get there.
