OTL's generally run a high output impedance, therefore, not a good choice for low impedance cans. Power is severely limited in many cases, too - because the output is directly driven from the tubes. Tubes are not known as heavy current producers - just the opposite. Transformer-coupled is better for low impedance, but the drawbacks are often some coloration or loss in low-end frequency response, unless $$$ is spent on the transformer and it's quality.
I'm biased, but this is one reason tube hybrids can be a good choice for low-impedance cans. You get all the advantages of tubes in the gain stage, but can couple the output with a slamming, discrete SS output stage, easily able to handle the lowest impedance headphones.
