How do autotransformers affect sound?

Just wondering, I've noticed many of the McIntosh amps have autotransformers.
1) Why have an autotransformer on a solid state amp? Is it because it gets around designing for different current draws from different speaker impedances?
2) For tubes amps it makes sense I guess. The Mcintosh tube amps can be paired to various different speakers even those with impedeances of 2 ohms (or anything between 1 and 16 ohms as McIntosh touts). Is the only reason many other tube amp designers don't do this because the autotramsformer is another component in the signal path? What is the trade off? I mean why not hook up a very nice tube amp through an autotransformer such as the Speltz one and use your favorite pair of low-impedance low efficiency speakers? Why rule all those out if there's a simple solution as an autotransformer.

As an example I'm wonder if I could hook up an MC2275 (100 watt tube amp) to my Aerial 7Bs (drops to 4 ohms in the bass region) and get good performance.

One thing I noticed in auditioning the Mcintosh integrateds the 6900 had smoother highs than the 6500 which I've heard was due to the autotransformer (hand-wound!).

I'm think about picking up an MC2275 or an MC252/402. I want to try tubes but don't want to change speakers right now.

regards, David