I have 14 pairs of NOS driver tubes, too many to name them all.
All of them have HUM noise with 300 ohm+ dynamic headphones, only the stock (driver) tubes, coming with the new revision of the amp ((17AX4GT) are quiet.
As long as you use planars or low ohm dynamics you won't have not much noise problems, but using high ohm dynamics is not a good idea.
The noise problems reported at the beginning of this thread still exist 1:1 and changing the driver tubes by Cayin for the new revision hasn't changed anything other than forcing you to use the stock tubes with high impedance HP's, or hope one day you will find a pair of NOS tubes that don't make any noise and at the same time bring the sound you like. - so good luck with that
imho, for users with high ohm dynamic HP´s, the HA-6A is not tube rollers dream, it´s a nightmare and in this case the amazing HA3 should be easy the amp of choice.
cheers