McIntosh autoformers vs direct-coupled output

Viridian, the X was Acoustats first speaker and it used the direct drive amps that clio mentions. They continued to use that amp in various models for two years but people wanted to use their own amps so Acoustat developed a two transformer interface which made the speakers reasonabley easy for most amps to drive. By the time my speakers, the model 2+2 came along the direct drive amps were no longer made. 
About 20 years ago I decided I could improve the performance of the speakers by doing several things. The first was designing an adjustable high voltage supply to the diaphragm. This allowed me to push the voltage right up to the point where the speakers started arching then back off just a little. Doing this gave me another 1000 volts. Acoustat had to design the speaker to tolerate the most humid environments. Up here in New England we are pretty dry and I run A/C in the house. Next I tossed the two transformers and their crossover and got one very big 1 to 100 transformer which favored high frequency response. Since I use sub woofers bass response did not matter to me. The overall result was a speaker that was 6db more efficient, more dynamic and I think more detailed. I also can not saturate these transformers without making my ears bleed. It is actually not the panels that limit headroom, it is the transformers. ESLs have a reputation for being fragile with limited headroom. That is not the case with properly designed ones at all. Sanders explains this nicely in his white papers and I like his speakers a lot. I just wish he would do a line source version. 

Well, you should see the monster transformer I have on my Acoustats and they for certain do not have that problem, at least when going in the opposite direction on ESLs. One also needs to keep in mind that a wire is a tube full of ping pong balls. You shove one in one end and another pops out the other end. This happens at the speed of light. By comparison music barely moves.  

That is exactly the purpose of the output transformer. To drop the output impedance normally of high impedance tubes to an impedance low enough to drive loudspeakers. SS outputs have low impedance to begin with and do not normally need a transformer. Mac uses the transformer to isolate it's output section from the load which I believe they feel enhances stability which enhances sound. They certainly do have a signature sound and it is not bad it is just not the most detailed, dynamic sound out there.

It is extraordinarily hard to blow a Mac amp. The transformers protect the amp from impedance irregularities and back EMF. Mac amps have this sort of floating smoothness for lack of a better term that many find attractive. On the speakers I use they are not as detailed or dynamic as a direct coupled class A amp but if reliability is your greatest concern Mac is the best.