Doesn't using zero transformers defeat the whole point of an OTL amp? It's putting a transformer in and makes it similar to a non OTL push pull tube amp.'Defeat' as used in the question above suggests that an OTL is trying to do something that is somehow in opposition to a normal amp with a transformer?
It isn't. What we are trying to do is get as close to the music as we can.
The fact is that the ZERO has a very low turns ratio. Because of this it has very wide bandwidth- wider than almost any amplifier made, tube or solid state. You can't do that with a regular tube amp with an output transformer. You need to start with an amp that already has a fairly low output impedance, and a circuit that is not putting DC through the transformer winding.
The ZERO is really a problem solver- for example, here in Minnesota a lot of people want to use the M-60 with Magnaplanars. Usually they have smaller Maggies, but the M-60 is usually challenged when presented with a 4-ohm load. The ZEROs fix that and you don't loose the transparency of the amp. My rule of thumb is that when using the ZEROs you don't encounter any serious sonic tradeoffs. If the amp sounds better without the ZEROs then you just don't need them.
I think its better if the amp doesn't need the ZEROs. That usually means the speaker load is 8 ohms or more as in the case of ozzy's speakers.