After a few more listening sessions, during which the amplifier (with feet and using 3.5mm input) has continued to function, I can, despite not being very analytical about such things, post a word or two more about its sound...
I find bass and lower mids to be luscious and delicious. I can't say accurate, because it is seductive and leaves me not caring about accuracy! It is so seductive that I listened to one particular rock album (Quicksilver Messenger Service: Happy Trails) every day for quite a few days.
However, I find upper mids (inc some vocal) a bit rough or piercing, making me reach out to lower the volume control, or even making my ears uncomfortable. That is fatiguing listening.
Of course, I'm not using the headphone as specified in the thread, and hope my comments on the amp are still useful.
I wonder if others found that the sound changed over time (insofar as one can judge such a thing, I do believe ears do quite a lot of the burning in)? Mine is a second-hand unit, so it has already been glowing a while, but I don't think it was used very much.