^^ I can explain this easily- I get this question a lot.
Actually the ZEROs are autoformers, with a very low turns ratios. They are used as problem solvers with a variety of amplifiers, not just our stuff.
One use that surprises many is that many transistor amplifiers benefit from the use of ZEROs when driving 4 ohms. This is because distortion in the amp is reduced. In high end audio, its usually about finesse rather than power.
In the case of an OTL, there are a number of cases where someone wants to use the amp with a speaker that might otherwise be a marginal or impossible combination. We used to sell our own outboard device years ago call the Z-Music transformer, but explaining why this was a good way to go became a lot easier when the ZERO came along, simply because it was not our own product!
In case there is any question, I like transformers a lot more than I like transistors. As audiophiles we are often presented with a situation where someone 'fell in love with a speaker' which might be a nice sounding speaker but not the best combination with a tube amplifier. At the same time they want to not just any tube amp but an OTL: problem solved.
BTW the effect of the ZERO is such that if the amp likes the speaker load, the ZERO will not be any help. An example is a set of M-60s on a set of Merlins. On a load where the amp is struggling (M-60 on a set of Magnaplanars for example) all of a sudden the ZEROs make a lot of sense. Due to their low turns ratio, the ZEROs actually have more bandwidth than the amps do, going from less than 3Hz to about 2MHz. This would simply not be possible with a normal OPT in a regular tube amp! The result is that the amp makes twice as much power into the 4 ohm load that it would otherwise, with lower distortion and longer tube life.
With MA-1s there is less benefit, so the ZEROs should be tried and the benefits weighed. MA-1s are much more comfortable with 4 ohm loads!