In spite of the huge tubes it is more of a neutral, tonally balanced and very resolving sounding amp than a lush, mid bass emphasizing, tubey sounding amp. There is plenty of power for any headphone you can throw at it. Mainline at about 600mWpc is pretty good most with inefficient cans, but some of the most inefficient headphones like K1000s or some of the HiFiMan headphones might not play clean as loud as they possibly can with a Mainline. That is no problem with the Neo, which can do about 4Wpc.
It also happens to make an incredibly good preamp, the only we we have come up with that can outdo the BeePre for the "you are there" kind of presentation.