Originally Posted by ninja380
So if the amp is set at 8 ohms and I hook up 6 ohms speakers it will be working harder to produce the same sound out if 8 ohms speakers....,?
This is true, if all other things are equal, which they almost never are. But for the purposes of this discussion, I would way "Yes, an amp with a 8/6 ohm switch can possibly work harder in the 8 ohm setting because it makes more power supply voltage available to the output stage.
Notice that I said, "can possibly", and not "always does". In the end a given amount of loudness comes from the same current flowing through the speakers regardless of the capabilities of the source (the amplifier).
Because the 8 ohm setting elevates the voltage available to the amp's output stage, it will make it run a little hotter. AFAIK, the difference is on the order of 10% or maybe a little more.
You've probably been educated to think that power amplifiers are delicate little flowers that need care and feeding to work well. In fact modern power amplifiers are like turbocharged cars. If you doddle around town and don't press the gas pedal down very far, they are mild-mannered. If you floorboard them, they have a lot of reserves to pleasure you with. Setting the 8/6 swtich to 8 increases your potential pleasure a bit.
And if I set the amp to 6 ohms then the volume will have to be turned up for the 8ohm speaker as the output is reduced?
No. The 8/6 ohm switch makes a minor change in the power output capability of the amp, not its gain. The volume control setting for a given amount of loudness will be the same. There's a pretty good chance that you can change this switch setting while the amp is operating and the results will either be a minor switching transient or nothing audible at all. When you go from 6 to 8 there will be a momentary current draw from the AC line to charge the power supply caps to a slightly higher voltage. When you go from 8 to 6 quickly, there should be very little observable difference. Please follow the manufacturer's recommendations about how to use this switch.
What the 8 ohm setting on an 8/6 switch does is let you turn the volume up a bit before the amp starts clipping. Would you hear the difference in a the best possible listening test? Not so much. The switch is there for the benefit of the test bench and conformance to FTC regulations. Call it a "Lawyer's switch". ;-)