Driving 1 ohm

Simontju is right on that one. The 2500A inherited the Acoustats. Long story short : I had Acoustat model 3 for a year and decided to have them rebuilt in 2+2 s. What I got back is something supposedly easier to drive but tremendously upgraded on the full range. And that brought me the concern. The rest of the electronics is probably forever : Simaudio Eclipse, CJ Premier 16,cables. The 2500A is here to stay until I get upgraditis again but for the time beeing, the WAF has been tested enough with the 2+2 s in the living room. I would not dare testing it again with large monoblocks,class A or tube amplifiers, at least for awhile. Class D maybe a solution but did not have the chance to audition some. In my vicinity(Canada), they remain quite scarce.

My Acoustat 2+2's do amazingly well driven by a PS Audio HCA-2 amp. Grab one here on Agon for $600 used.. you won't be sorry, and your back will thank you!

If they were Apogees or Kappa9's, a rugged SS amp would be the way to go but for ESL's, Ralph is right.

Another option is "monostrapping" or parallelling an identical pair of stereo amps. That would double the current and wattage and halve the output impedance. Some manufacturers don't approve of this method.

Rwwear, in the real world, people want to use OTLs with low impedance speakers. If you use an autoformer to do this, you often meet with success. The fact of the matter is that the autoformer has bandwidth that is wider than most amps, tube or solid state, which is only possible because its turns ratio is so low.

IOW, using the ZERO would not be practical if you did not already have a low output impedance and no DC at the output. So that could be an OTL, but it could also be a transistor amp. Its already been shown that a transistor amp will sound better driving 4 ohms or less if it does so through a set of ZEROs rather than direct.

So the argument holds regardless of the amplifier technology, which inherently includes OTLs.

For $2000, the H2O Audio Signature 100 can drive a 1 ohm load with no sweat. Hell, they can handle the vaunted Apogee Scintilla with absolute ease. There aren't too many (great sounding) and truly affordable power amp's that can make such a claim.