Well, yes, the HeadSix is OTL, but that's not quite the point I was making.
OTL is shorthand for "Output TransformerLess," or that there's no output transformer on the amp.
Solid state doesn't use output transformers because chips have a very low output impedance. McIntosh and a few others used output transformers on chips, but you rarely see that type of circuit used.
Tubes, on the other hand,tend to have a high output impedance. So if you don't use output transformers on a tube amp (OTL), then you get high output impedance which makes things wonky when you plug in headphones that have an impedance that's less than the output impedance of the amp.
The best way to lower output impedance on a tube amp is to use output transformers. However, those aren't cheap and add to the cost significantly. This is why the cheap tube amps are all OTL, and they struggle with low impedance headphones.