More questions...

What do the impedance switches on the Torii MKIV do?  I get the general concept, and that one setting may sound better than the other.  In my system I prefer the 4 ohm setting (switch to the back) and notice that it produces about a 3dB increase in SPL when measured with pink noise and a dB meter.  

I don't know why though.  Most amplifiers will actually output more watts in the 8 ohm setting.  The 4 ohm setting usually steps down the rail voltage fed to the amplifier to prevent possible overheating.  

Can someone explain why the 4 ohm setting is louder in this amplifier, when that switch traditionally cuts the rail voltage in most amps?  Thanks!

Yes, like Jim I don't have the knowhow to explain that. I find that the position to the back of the amp seems to offer more "loudness" to me than the position with the switch back towards the front of the amp, but the frequency response is different too and I don't have a measuring tool to tell which is actually "louder." I prefer the switches pointing towards the front of the amp, it's a bit less forward, I'm not a fan of forward. But with some material the other position is an improvement. I enjoy having both options. But my speakers are 4 ohm so that makes sense.

Good question.  I am using 4 Klipsch Heresy's in two channels.   They are nominally 8 ohms, but since there are two on each channel, I would characterize them as a 4 ohm load.  

Thanks all for the responses!

The Torrii mk2 manual says higher impedence is toward the back so the mk3 is most likely the same.  The mk4 must be a departure from the previous norm.