McIntosh autoformers vs direct-coupled output

stereo5, you have repeatedly described the parasound halo series as thin and lifeless. I am not sure what your other components were that you paired it with, but I disagree with your subjective evaluation and think you do a disservice to others who may be seeking opinions on what amps to purchase who might pass on these very good amps.

Viridian:
"Don't all opinions mask subjective bias"?
Of course! Sorry. I might have simply asked for factual technical info to
help me determine a direction.
Georgehifi: M.L. #3 - powered woofer.
To all: I accept the notion(s) that there are better amp choices
available, but I have a limited budget and have a DeWick re-built C-37
which offers the low-level sound shaping I require.  Unless there there is a compelling cost/performance disparity, I'd prefer to continue with a
Mac amp. Now, given my answer to George, what Mac amp - Auto,
or direct, and what power - would be the most suitable, least
compromising pick? 
Most appreciative of your input - all of you!
Bo

@broockies, 

It all ends up what sound signature you like listening to.  Two similarly measured amps can sound different with the same speakers.

As George mentioned, the ML panels go below 1ohm in the higher frequencies.  From my experience, the amount of power needed to bring out the full potential of the panels is related to the size of the panel itself.  What could easily drive my older Source MLs speakers, sounded anemic with the 11As.  

With MLs if you want a crisper sound signature go with direct coupled.  If you want warmer sound with more rolled off highs then go with autoformer.  If going directly coupled, my recommendation would to make sure the amp is rated down to at least 2ohm.  Otherwise, go with autoformer to make sure you provide enough stable power to the speaker.

I do believe that matching preamp and amp is the way to go.  Only reason I mentioned alternatives is that a used McIntosh amps sell far higher on the used market than far newer competitive brands.

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.