A lot of what people perceive being "typical tube sound" is owed to tansformers in the signal path (as such, just to achive exactly this, some amp companies put transformers even into a solid state signal path. check out the difference between JR Capri 1 and Capri 2). OTL tube amps do not have that transformer sound signature. Still, tubes are said to produce more even harmonics than transistor amps and by such sound more natural... but I would say this is history. Modern well designed tube amps as well as transistor based amps have very very little distortion anyway; and the less, the more similar they sound... if(!) the tube amp is ideally matched to the headphone impedance wise. So this last point might be the biggest difference. The damping factor. Here tube amps react more sensitive on the matching. And sometimes, with tube amps more than with transistor amps, it leads to a emphasis on bass that sounds good on otherwise too thinly sounding cans.... but this is a compensation of mistakes, which from my point of view should not be the way (I also do not like to equalize).
