MAC Autoformers?

So here is my question. What is happening electrically if there is an impedance mismatch? Is the amp producing distortion if there is an impedance deviation from 3000 ohms? If so, does having larger wattage and power supply capabilities ameliorate distortion effects?  

How about negative feedback?  In my case, I understand that the Ref 150 uses 14db of negative feedback and as a result, performs like a voltage source amp. Does that feature help to reduce distortion effects?  

I have not touched on phase angle or sonic coloration issues caused by impedance mismatches between the amp's output taps (nominally, the 4, 8 or 16 ohm taps) and the speaker's actual impedance at a particular frequency.  

As to sonic coloration, I recall that the output impedance of the ARC Ref 150 off the 8 ohm taps is around 1 ohm or less. This results in some sonic coloration, ... but not a lot. In a Stereophile review of the ARC Ref 150, John Atkinson measured the Ref 150's voltage output variation off the 8 ohm taps to be (+) or (-) .8 db when presented with a synthetic speaker load.  

Btw, are similar issues presented with OTL tube amps?

@bifwynne 

All amps always make distortion. If the amp's tubes are loaded below 3000 ohms, distortion will rise. If loaded above that, its power will decrease as will its distortion. 

The feedback allows the amp to behave as a voltage source. George has it wrong; the amp can make double the power into 4 ohms as it does into 8 because voltage sources do that (this is by definition). The deal is, the 4 ohm power is the maximum power for which the amp is rated and you have to be on the 4 ohm tap. If the impedance then rises to 8 ohms, the amp will make 1/2 of its output power. Its a bit confusing, because tube amps accomplish the task of being a voltage source in a way that is opposite of how solid state amps do it.

The issues are the same with an OTL, but of course no transformer is (usually) involved. Instead the tubes see the load directly.

@bifwynne I run into that too- we make OTLs as you know, but there are speakers out there that on paper would seem to be too difficult a load, but in practice work out just fine with our amps. So your experience does not surprise me in the least.

Feedback is helping you out here, and my surmise is that most of the speaker impedance curve is actually higher than the dips in the bass (the bass region also has peaks BTW).  That's a pretty simple explanation for why this is working for you. 

Quite often the 4 ohm tap on an output transformer sacrifices some performance- in some cases you can lose an octave of bandwidth off the bottom just by going from the 8 ohm tap to the 4 ohm tap. So it can be a mixed bag in the real world (as opposed to the math world of theory).

I use I am using a Mac C2500 tube preamp mated to a Mac MC 152 amp with the transformers driving GE Triton Reference speakers and this combination has gotten me off the merry go round as it is the best sound I have ever had in my listening room.  

My other system uses. Mac MA6600 Integrated with transformers.  I was considering a MA6500 but Audio Classics who know more about Mac than anyone else with the exception of McIntosh told me I would lose the lush romantic Mac house sound without the transformers. Needless to say I am thrilled with all my Mac purchases and am done.