T_bone, you got it exactly right. Most SS amps make constant voltage with respect to load; on ESLs that is not the right response.
With respect to the ZERO/OTL thing, we made a device similar to the ZERO about 10-15 years ago. It got to be a bit of a chore explaining why it did not negate the 'OTL character', so eventually we stopped. Paul Speltz picked up the slack.
Here is the explanation: OTLs already have low output impedance and do not make DC, so some of the bigger requirements transformers face have been reduced. To start with, the device does not have to block DC, so its an autoformer with no secondary. This is a lot simpler. 2nd, we're only going from 16 ohms down to 2,3 or 4 ohms, so the turns ratio and distributed capacitance is such that the device has bandwidth out to 1MHz, something that is impossible for a regular transformer. In fact the technology only works if you have a low output impedance and no DC- so its fine for an OTL- particularly in the light of a speaker load that the amp would not work with otherwise! Finally, in the case of our amps, you do not negate the triode-class-A-single-stage-of-gain-fully-differential-zero-feedback aspects. So it is a nice solution. Paul sells a lot to the single-ended crowd as well.
With respect to the ZERO/OTL thing, we made a device similar to the ZERO about 10-15 years ago. It got to be a bit of a chore explaining why it did not negate the 'OTL character', so eventually we stopped. Paul Speltz picked up the slack.
Here is the explanation: OTLs already have low output impedance and do not make DC, so some of the bigger requirements transformers face have been reduced. To start with, the device does not have to block DC, so its an autoformer with no secondary. This is a lot simpler. 2nd, we're only going from 16 ohms down to 2,3 or 4 ohms, so the turns ratio and distributed capacitance is such that the device has bandwidth out to 1MHz, something that is impossible for a regular transformer. In fact the technology only works if you have a low output impedance and no DC- so its fine for an OTL- particularly in the light of a speaker load that the amp would not work with otherwise! Finally, in the case of our amps, you do not negate the triode-class-A-single-stage-of-gain-fully-differential-zero-feedback aspects. So it is a nice solution. Paul sells a lot to the single-ended crowd as well.