Now testing the Teac NT-505 headphone output, versus the output from my dedicated headphone amp, Audiotail Jade OTL.
I am surprised by the diffence. Yes, the 505 is quite good, acceptable. But Jade is much better. Regarding musicality, authority, timbre - and other aspects, clearly heard.
Note that, I have two good Nos tubes in the Jade - RCA for the output tube, and Telefunken for the driver 12ax7. These lift the performance of the Jade. With stock new production tubes the gap would probably be a bit smaller.
My test, now, is limited to Audioquest Nightowl headphones. Maybe other earphones would give different results - though I doubt it - remains to be tested.